Problem: In multisig.mligo, else clause on line 36 ends on line 55,
while it should end on line 80. This happens because of incorrect
lambda parsing on lines 42-43. In particular, `let valid, keys = vk`
in lambda body gets parsed as a
`tuple (fun_app "let" "valid") (binary_op "=" "keys" "vk")`, and `in`
is considered as a part of the outer `let`.
Solution: Allow `_program` instead of `_expr` as the lambda body.
Problem: It happens that "go to definition" fails with lambda
parameters meaning that we do not create proper AST for it.
Also we need to restructure grammar so that we can support it in
the future.
Solution: Restructure grammar to use "param_decl" and ensure that
we use proper fields for parser resolution.
Problem: Before making parser for ReasonLIGO we need to construct
its grammar so that we can collect *all* errors and simplify our
further work with AST. Also since we use those grammars in squirrel
it would be conscise to use them from the project directory directly.
Solution: Add grammar for ReasonLIGO and move grammar to squirrel.
