Matching/ArgThat to accept eval() strings

[Xion]

Essentially:

ArgThat('x % 2 == 0')

Caveats:

• Scoping. It's probably best if the compiled closure didn't include local scope of the matcher ctor invocation, but it should still include global scope (for imported modules).
• Name of the placeholder/argument variable could be deduced intelligently (it'd be the sole unbound variable), but it probably requires tricks with the Python code object.