[gobwas]

glob: introduce Compiler with added Globstar option

Compiler holds the compilation options; Compile(pattern, seps...) is
sugar for Compiler{Separators: seps}.Compile(pattern).

With Globstar, a `**` that is a whole segment and is followed by a
separator matches zero or more segments: `**/x` matches `x`, `a/**/b`
matches `a/b`. It is a parse-time rewrite into the `{**/,}` idiom, so
the matching engine is unchanged. The fuzz oracle learns the same rule,
and the fuzzer now takes arbitrary separators.

Closes #76. See also #58, #65.