Compiler optimisations!

[HARD] Immediate operations for values that support it (up to +/- u16::max for addi and subi respectively)
- this requires significant complexity in code generation as we need to traverse down the tree when we come across these operations to prevent additional register allocations.
[EASY] Add multiply and divide operations to code generation
[MEDIUM] proper prefix/postfix inc/dec implementation. slightly more complex as we need to check for a variable and modify it in place
[EASY] Investigate logical and operator not compiling - either a lexer or parser issue.
[MEDIUM] Get shift operations working correctly.