add lambda support to Unused.*Linter

[jjergus]

Fixes #341

• improves some other cases (unused params/variables inside lambdas are caught)
• some false negatives still exist (using a shadowed param/variable inside a lambda also marks it as used in the enclosing function) -- probably fixable but I didn't want to do too much in one PR

[jjergus]

@ryangreenberg I can't add you as a reviewer but you might also be a good person to review this.

[ryangreenberg]

Nice addition! I think your use of traverse() in a few places makes it easier to read and less dependent on knowing the exact hierarchy of the AST.

[ryangreenberg]

For a future change, I think this is roughly where we want to check that $node is a child of $functionish, not any descendant to fix #341