Allow optimization of bytecode

[lausek]

Constructs that require labels

• expressions for logical short-circuit (And, Or)
• branches
• loops (break, continue)

Optimization cases

• [X] constant expressions like 1 + 2 are not optimized:

10:    Pushc(0)
11:    Pushc(1)
12:    Add

• [X] generator could merge these expressions in Branch conditions into Jt:

10:    ...
11:    Not
12:    Jf
13:    ...

• [X] the generator often emits code like this at the end of Branch (doesn't happen anymore after introduction of LIR?):

25:    ...
26:    Jmp(27)
27:    ....

• [x] dead code elimination. everything after offset 12 cannot be reached #9:

func:
11:    Pushc(0)
12:    Ret
13:    Pushc(1)
14:    ...

Steps for implementation

• [x] HIR is a confusing name. change it to HirUnit (?)
• [x] update code to use module_builder.entry() instead of module_builder.add(ENTRY_POINT)
• [x] update docs
• [X] create new type Label
• [X] create new type LirElement
• [X] rename current LoweringRuntime to HirLoweringRuntime that transforms HIR -> LIR
  • [X] on each emit try to optimize the last instructions (depending on instruction)
• [X] create a new runtime LirLoweringRuntime that transforms LIR -> Bytecode