Decrease SP and BP after def'ing a symbol.
Assembler: support explicit specifications of memory access sizes.

The syntax is [mul [:word :bp]] or [mul [:byte :bp]]. Saying just
[mul [:bp]] defaults to word multiplication.
Compiler: explicitly specify word size in * and / primitives.
Make write-char primitive expect a Lithium integer.
Introduce global-env-start into the compilation state, assign memory for global def'd variables from there.