Add purpose comments to some test contracts

Add comments explaining what a contract is/does/tests to the top of several PascaLIGO test contracts, as part of evaluating what parts of the syntax are and aren't currently tested.

src/contracts/annotation.ligo

@@ -1,3 +1,5 @@
+// Test that type annotations work in PascaLIGO
+
 const lst : list(int) = list [] ;
 
 const address : address = "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx" ;

src/contracts/arithmetic.ligo

@@ -1,3 +1,5 @@
+// Test PascaLIGO arithmetic operators
+
 function mod_op (const n : int) : nat is
   begin skip end with n mod 42
 

src/contracts/bitwise_arithmetic.ligo

@@ -1,3 +1,5 @@
+// Test PascaLIGO bitwise operators
+
 function or_op (const n : nat) : nat is
   begin skip end with bitwise_or(n , 4n)
 

src/contracts/boolean_operators.ligo

@@ -1,3 +1,5 @@
+// Test PascaLIGO boolean operators
+
 function or_true (const b : bool) : bool is
   begin skip end with b or True
 

src/contracts/closure-3.ligo

@@ -1,3 +1,7 @@
+// This might seem like it's covered by induction with closure-2.ligo
+// But it exists to prevent a regression on the bug patched by: 
+// https://gitlab.com/ligolang/ligo/commit/faf3bbc06106de98189f1c1673bd57e78351dc7e
+
 function foobar(const i : int) : int is
   const j : int = 3 ;
   const k : int = 4 ;

src/contracts/condition-simple.ligo

@@ -1,3 +1,5 @@
+// Test if conditional with trivial conditions in PascaLIGO
+
 function main (const i : int) : int is
   begin
     if 1 = 1 then

src/contracts/condition.ligo

@@ -1,3 +1,5 @@
+// Test if conditional in PascaLIGO
+
 function main (const i : int) : int is
   var result : int := 23 ;
   begin

src/contracts/declaration-local.ligo

@@ -1,3 +1,5 @@
+// Test PasaLIGO variable declarations inside of a block
+
 function main (const i : int) : int is block {
   const j : int = 42 ;
 } with j

src/contracts/declarations.ligo

@@ -1,3 +1,5 @@
+// Test PascaLIGO top level declarations
+
 const foo : int = 42
 
 function main (const i : int) : int is

src/contracts/error_type.ligo

@@ -1 +1,3 @@
-const foo : nat = 42 + "bar"
+// Test that PascaLIGO will reject a type declaration with improper value expression
+
+const foo : nat = 42 + "bar"

src/contracts/function-complex.ligo

@@ -1,3 +1,5 @@
+// Test a PascaLIGO function with more complex logic than function.ligo
+
 function main (const i : int) : int is
   var j : int := 0 ;
   var k : int := 1 ;

src/contracts/function-shared.ligo

@@ -1,3 +1,5 @@
+// Test a PascaLIGO function which uses other functions as subroutines
+
 function inc ( const i : int ) : int is
  block { skip } with i + 1
 

src/contracts/function.ligo

@@ -1,3 +1,5 @@
+// Test a trivial PascaLIGO function
+
 function main (const i : int) : int is
   begin
     skip

src/contracts/heap.ligo

@@ -1,3 +1,6 @@
+// Implementation of the heap data structure in PascaLIGO
+// See: https://en.wikipedia.org/wiki/Heap_%28data_structure%29
+
 type heap is map(nat, heap_element) ;
 
 function is_empty (const h : heap) : bool is

src/contracts/high-order.ligo

@@ -1,3 +1,5 @@
+// Test a PascaLIGO function which takes another PascaLIGO function as an argument
+
 function foobar (const i : int) : int is
   function foo (const i : int) : int is
     block { skip } with i ;

src/contracts/included.ligo

@@ -1 +1,3 @@
+// Test PascaLIGO inclusion statements, see includer.ligo
+
 const foo : int = 144

src/contracts/includer.ligo

@@ -1,3 +1,5 @@
+// Test PascaLIGO inclusion statements, see included.ligo
+
 #include "included.ligo"
 
 const bar : int = foo

src/contracts/list.ligo

@@ -1,3 +1,5 @@
+// Test list type and related built-in functions in PascaLIGO
+
 type foobar is list(int)
 
 const fb : foobar = list

src/contracts/loop.ligo

@@ -1,3 +1,5 @@
+// Test while loops in PascaLIGO
+
 function counter (var n : nat) : nat is block {
   var i : nat := 0n ;
   while (i < n) block {

src/contracts/map.ligo

@@ -1,3 +1,5 @@
+// Test map type and related built-in functions in PascaLIGO
+
 type foobar is map(int, int)
 
 const fb : foobar = map

src/contracts/match.ligo

@@ -1,3 +1,5 @@
+// Test the pattern matching functionality of PascaLIGO
+
 function match_bool (const i : int) : int is
   var result : int := 23 ;
   begin

src/contracts/multiple-parameters.ligo

@@ -1,3 +1,5 @@
+// Test functions with several parameters in PascaLIGO
+
 function ab(const a : int; const b : int) : int is
   begin skip end with (a + b)
 

src/contracts/option.ligo

@@ -1,3 +1,5 @@
+// Test the option type in PascaLIGO
+
 type foobar is option(int)
 
 const s : foobar = Some(42)

src/contracts/record.ligo

@@ -1,3 +1,5 @@
+// Test record type in PascaLIGO
+
 type foobar is record
      foo : int ;
      bar : int ;

src/contracts/set_arithmetic-1.ligo

@@ -1,3 +1,5 @@
+// Test set iteration in PascaLIGO
+
 function iter_op (const s : set(int)) : int is
   var r : int := 0 ;
   function aggregate (const i : int) : unit is

src/contracts/set_arithmetic.ligo

@@ -1,3 +1,5 @@
+// Test set type and basic operations in PascaLIGO
+
 const s_e : set(string) = (set_empty : set(string))
 
 const s_fb : set(string) = set [