NodePattern: Add Constants and keyword parameters

[marcandre]

This PR allows use of constants in a NodePattern, and allows keyword parameters (in addition to the existing sequential parameters).

Example from the main gem

-       OPS = %w[== === != < > <= >= <=>].freeze
+       OPS = %i[== === != < > <= >= <=>].to_set.freeze

        def_node_matcher :useless_comparison?,
-                        "(send $_match {:#{OPS.join(' :')}} $_match)"
+                        "(send $_match %OPS $_match)"

This would also be faster (doing a single hash lookup instead of 8 comparisons, see #29)

The keyword parameters is for cases where positional parameters wouldn't be as clear, but also because I would like to make it possible to specify some keyword parameters at the definition too (upcoming PR)

For example, if one doesn't care to define a constant, one could write:

       def_node_matcher :useless_comparison?,
-                         "(send $_match {:#{OPS.join(' :')}} $_match)"
+                         "(send $_match %ops $_match)",
+                         ops: %i[== === != < > <= >= <=>].to_set.freeze

This could be overridden when calling the matcher, e.g.

useless_comparison?(node, ops: ...)

[marcandre]

PS: Still missing doc for %keyword_parameter form.

/cc @bbatsov

[marcandre]

PR finished, defaults added to def_node_pattern/search, documentation added too.

[bbatsov]

Might be a good idea to accept arrays here as well and just implicitly convert them to sets. Seems to me this will result in nicer end user API, if it's more flexible.

[marcandre]

Aren't you aware of my machiavelic plan to convert the whole world to Sets? ;-)

I like that currently the API is simple, we use <your object> === <node element>

It would be easy to convert the values to Sets at the level of def_node_matcher, but it would be a bad idea to do it at the matcher call level, so I'm afraid there would be confusion...

I should add a note in the doc that Array#=== will never work on a AST element.

[marcandre]

Doc amended on master

[marcandre]

Changelog entries added, PR ready as far as I'm concerned.

[marcandre]

Rebased.