Allow 'n' and 'f' value modifiers to double as decimal points

This enables XnY style microtonal note number shorthand, instead of the
usual X.Yn format.

Also refactored a2_GetNum() a bit.

Added microtonal.a2s.

Closes #176.
Fail if a numeric literal is directly followed by a number, letter, or '.'

We only want whitespace, ';', ',' or an operator character directly
following a numeric literal, because anything else is most likely a typo
that will result in weirdness if allowed to parse "correctly."

Closes #175.