Optimize SSA CFG datastructure + construction

[clonker]

• replace std::variant for SSACFG::ValueId by tagged union
• replace std::deque in SSACFGBuilder by a std::vector, possible by carefully reording operations in recursive variable definition chain
• make ssacfg json serialization stateless (it was stateless already but wrapped in a class)

Makes the ssa cfg builder 26.5% faster.

[blishko]

Looks good to me, I've just left a few small comments.

[blishko]

LGMT!