Add Parser.find_method()/find_methods()

[tabatkins]

Currently, widlparser has Parser.normalized_method_name(), which will search for a method based on the args (but doesn't understand the Foo/bar path syntax), and Parser.find(), which understands the path syntax but ignores arguments.

I was trying to fix speced/bikeshed#1192, and realized the issue was that I was relying on both of these methods having both of the abilities, meaning I was hitting error cases both times!

This PR fixes three things I ran into while debugging this:

1. Adds support for path syntax in normalized_method_name, if you don't manually pass the interface name as a separate argument. That is, Foo/bar(arg) will now work, rather than falling down to the final error case because there's no method named Foo/bar.
2. Makes find() and find_all() use a consistent ordering. Not strictly required for this, but it bugged me that find() gave the last result in the parser while find_all() gave them in order. Generally, foo() should be equivalent to foo_all()[0]. I can snip this commit out if you really want.
3. Adds a find_method() (and find_methods()) method to Parser, which uses the full parser-normalization and argument usage of normalized_method_name, but returns the actual matching construct.

I'm considering making find() and find_all() dispatch to find_method() if it finds any parentheses, and only doing its current "remove/ignore parens and just look for a name" if that fails to find anything. I haven't added that in yet, tho.

I also haven't run the linter yet, apologies. Just wanted to get the patch up for an eye-over before I left for the day.