feat[venom]: offset instruction

[HodanPlodky]

What I did

Created offset handling for statically resolvable values

How I did it

I introduced offset instruction that is emited in algebraic pass when the add instruction calculates offset with literal values

How to verify it

Created new test in algebraic pass tests

Commit message

this commit introduces an `offset` instruction that is emitted in the
algebraic pass when the add instruction calculates an offset from a
code label, which is used for immutables. this allows compilation
directly to the magic `OFST` assembly instruction, which does
additional constant folding after symbol resolution.

[charles-cooper]

Suggested change

	if inst.opcode in ["store", "offset"]:
	if inst.opcode in ("store", "offset"):

[charles-cooper]

i guess this works so long as this pass runs after the store elimination pass

[charles-cooper]

i'm seeing about a 1% reduction in bytecode size for some contracts which use immutables :)