Compiler/VM improvements

Syntax for local 'def' changed to "def .SomeSymbol SomeValue".

Implemented (<expr>) ... syntax in the "times," send and spawn constructs.

Implemented SENDR, SPAWNV and SPAWNVR instructions to support the above.

Implemented KILLR instruction and support for 'kill' with expressions.

Variables can now be used without parentheses as voice IDs for the ':'
(spawn) and '<' (send) operators.

Closes #7, as well as a few issues that never made it out of the TODO
or inline FIXMEs.