Improve set handling

[jaynetics]

This makes CharacterSet a standard Subexpression as suggested in #47 (comment)

All equivalent tokens result in the same Scanner and Parser emissions as outside of sets.

New CharacterSet::Range and CharacterSet::Intersection expressions represent respective trees.

Other notable changes are:

example	from type, token	to type, token	from exp	to exp
[\b]	:set, :backspace	:escape, :backspace	none/String	ES::Backspace
[[:xy:]]	:set, :char_xy	:posixclass, :xy	none/String	PosixClass
[[:^xy:]]	:set, :char_nonxy	:nonposixclass, :xy	none/String	PosixClass
\x20	:escape, :hex	:escape, :hex	ES::Literal	ES::Hex
\x20	:escape, :octal	:escape, :octal	ES::Literal	ES::Octal
\u1234	:escape, :codepoint	:escape, :codepoint	ES::Literal	ES::Codepoint
\u{12 34}	:escape, :codepoint_list	:escape, :codepoint_list	ES::Literal	ES::CodepointList

@ammar What do you think? The commit messages provide a bit more explanation if you are wondering about some of the changes, but feel free to suggest any other solution.

[jaynetics]

Turns out I should have read the docs...
https://github.com/k-takata/Onigmo/blob/79114095/doc/RE#L155-L156

Intersections apply to all expressions in their set, not just adjacent ones.

'abc1'.scan(/[a b \d && b c [:digit:]]/x) # => ["b", "1"]
'abc1'.scan(/[^a b \d && b c [:digit:]]/x) # => ["a", "c"]

So maybe Intersection parse results need to look somewhat like this:

RP.parse(/[a&&b]/).first.first # =>
  #<Intersection @expressions=[
    #<Intersection::Left @expressions=[
      #<Literal @text="a"/>
    ],
    #<Intersection::Right @expressions=[
      #<Literal @text="b"/>
    ]/>
  ]/>

Now that would require quite a bit of tree restructuring while parsing.

Not to mention that there can be more than one intersection:

'abc1&'.scan(/[abc && ab && bc]/x) # => ["b"]

Another option could be to treat Sets as group of Sequences by default, which, however, might make them harder to handle just for this somewhat exotic feature.

Hmmm ...

[jaynetics]

I'm reasonably happy with this now ...