version: add new Version::Parser class

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This PR adds four new classes for parsing version strings from Pathnames: Parser, RegexParser, UrlParser, and StemParser

• Parser is the base abstract class and currently does nothing; all it contains is an abstract parse method.
  • (This class is useful to have if we ever need to parse version strings through means other than regular expressions.)
• RegexParser is a sub-class of Parser and uses regular expressions to extract version strings from Pathnames.
  • This class is also abstract; sub-classes of RegexParser must implement the process_spec method.
  • It accepts an optional block that is called to finalize the parsed version string; this is used for Boost version styles to replace underscores (_) with dots (.)
• UrlParser is a sub-class of RegexParser that parses the full Pathname (i.e. URL) string.
• StemParser is a sub-class of RegexParser that only parses the stem of the Pathname (i.e. the base file name)

[BrewTestBot]

Review period will end on 2021-01-21 at 22:58:36 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Yeh, that's dramatically cleaner, nice work!

[MikeMcQuaid]

Any unit tests for these would be lovely but non-blocking.

[MikeMcQuaid]

👏🏻 ❤️