Replace Value._wrap with a less general, less crazy solution.
Remove support for bitops, which Sass lacks syntax for.
Remove __cmp__ and __rcmp__; former unnecessary, latter obsolete.
Remove ParserValue support from types which no longer use ParserValues.
Remove convert_to.

Only used by the unit conversion pseudo-functions, which have since been
replaced by parser improvements.  Many of these implementations made no
sense and could never be called by the parser anyway.
Remove some _undefined_re checks from type system.

Literal 'undefined' strings should no longer be possible, now that the
parser recognizes them and there's a null type.
Simplify @if handling code a bit.

Removes another lingering _undefined_re.
Some expression evaluation tests.  WIP
Start on tests for Compass helpers.

Fixed some bugs while I was at it.

- Boolean conversion was totally wrong; Sass uses Ruby rules.
- blank() was wrong.
- compact() was needlessly complicated.
Eliminate _undefined_re once and for all.