Add regexp module

[myzie]

Adds support for regular expressions:

• New object.Regexp Risor type that wraps *regexp.Regexp.
• New regexp module

The regexp module object is callable, so regexp("foo") returns a compiled Regexp object.

go run ./cmd/risor
Risor

>>> r := regexp("foo.?")
>>> r
regexp("foo.?")
>>> r.match("food")
true
>>> r.match("fo")
false

[edhemphill]

package regexp