SSA CFG: stack to memory spilling

[clonker]

Implements stack to memory spilling for the SSA CFG pipeline in a simple stupid version.

Adds:

• ssa/spill/MemoryAddressing is a per-subobject owner of spill memory
  • sums numSpilled() across each CFGs SpillSet and bumps memory guard accordingly, allocates non-overlapping memory addresses for spilled values
• ssa/spill/Emitter emits bytecode against the symbolic stack for spilled values
• ssa/spill/SpillSet got a new method feasilize which will check for all spilled values whether they can be feasibly spilled at the point of origin, may add more things to spill accordingly

Modifies

• ssa/StackLayoutGenerators generate() to return {layout, spillSet} instead of just the layout and also validates back-edges asserting that back-edge targets can be safely shuffled to
• ssa/CodeTransform is reworked to first generate per-CFG layouts and spill sets (feasilize them), then build a single global MemoryAddressing to hand into the CodeTransform instance

Test

• Adds SpillTest suite under test/libyul/ssa/spill which runs the generate -> feasilize -> address sequence and emits that plus a text representation of the input object. does not generate any code.