Refactor/rename source locations, tracking, etc

[milseman]

Track source locations for Atoms, quantifier sub-values (like ranges and modifiers), and other refactoring and changes.

Naming convention:

• Source.Position: A dimensionless point in the input
  • Corresponds to an index in the input
• Source.Location: The region of the input from which a parsed entity came
  • Corresponds to an index-range in the input, but separate type to hold API
  • SourceLocation typealias
• AST.Loc / Source.Loc: a located value, that is a value with a location.
  • Name is short because it's ubiquitous

AST builders and faking:

• Builder global functions internalized and sunk into the _StringProcessing module
• _fake globals removed in favor of static members or inits on SourceLocation
• atom_m makes the custom char set member variant instead of the AST variant

Changes to AST or parser functionality:

• Trivia now records the contents as a String
• Source is no longer a collection, and dropped the Peekable protocol.
• Atoms tracked
• Quantifier values (like the ranges or kinds) tracked

After this PR, there's only a handful of minor things to track:

• The | in alternations
• Potentially, even finer-grained pieces:
  • The - in ranges inside custom character classes
  • Such as the closing ) tracked separately, though that's derivable from the group's range
  • Quote's \Q and \E distinct from the contents of the quote
  • The location of the ^ for inverting Unicode properties inside a \p{^...}
  • The portion covering the name in named back references (groups do have it though)
  • The location of the comma in a quantification a{3,7}
  • ...

[rxwei]

Suggested change

	mutating func recordLoc<T>(
	mutating func recordLocation<T>(

[milseman]

Actually, this shouldn't even be internal. I'll make it fileprivate to lexical analysis. Within fileprivate usage, I think the abbreviation is appropriate.

[milseman]

@swift-ci please test linux platform