- ruby*.y: fixed context inside lambda args and module.

[iliabylich]

Previously we didn't push context for module and lambda arguments and so we didn't reject some code (or we rejected what's allowed).
However, examples that should've raised errors are complicated and it's very unlikely that any code was affected. At least no related issues were reported.

Examples of code that is accepted/rejected differently after this change:

1. ->(a) { module M; _1; end }
   We didn't push context for module M, and so argument a conflicted with implicit parameter _1. In Ruby module opens a new scope and so _1 is just a regular local variable.

2. class A; ->(a = (return; 1)) {}; end
   This code is valid, because return is a part of the lambda. But we didn't store this fact internally, and so it was rejected before.

Both examples described above (and all similar variations) are fixed in this commit.

Also, I've slightly improved assertions related to context (assert_context). Instead of checking state of the half-reduced AST (which is impossible for lambda args for example) we use a special get_context macro-like node that grabs snapshot of the parser.context.