Add SUBRK and DIVRK bytecode instructions to bytecode v5

[zeux]

Right now, we can compile R*K for all arithmetic instructions, but K*R gets compiled into two instructions (LOADN/LOADK + arithmetic opcode).

This is problematic since it leads to reduced performance for some code. However, we'd like to avoid adding reverse variants of ADDK et al for all opcodes to avoid the increase in I$ footprint for interpreter.

Looking at the arithmetic instructions, % and // don't have interesting use cases for K*V; ^ is sometimes used with constant on the left hand side but this would need to call pow() by necessity in all cases so it would be slow regardless of the dispatch overhead. This leaves the four basic arithmetic operations.

For + and *, we can implement a compiler-side optimization in the future that transforms K*R to R*K automatically. This could either be done unconditionally at -O2, or conditionally based on the type of the value (driven by type annotations / inference) -- this technically changes behavior in presence of metamethods, although it might be sensible to just always do this because non-commutative +/* are evil.

However, for - and / it is impossible for the compiler to optimize this in the future, so we need dedicated opcodes. This only increases the interpreter size by ~300 bytes (~1.5%) on X64.

This makes spectral-norm and math-partial-sums 6% faster; maybe more importantly, voxelgen gets 1.5% faster (so this change does have real-world impact).

To avoid the proliferation of bytecode versions this change piggybacks on the bytecode version bump that was just made in 604 for vector constants; we would still be able to enable these independently but we'll consider v5 complete when both are enabled.

Related: #626