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: 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.
