Use constexpr arrays for metadata about IL instruction operands

[bdash]

This avoids constructing numerous unordered maps and vectors at library load time in every plug-in that uses the C++ API. Additionally, the arrays allow for more efficient look-ups.

[plafosse]

In general I'm a huge fan of this PR. I appreciate all the static asserts which should help this code from breaking in the future. I don't see downsides to any of this. This seems like it should shave off some bytes on every IL instruction allocation which is a nice win too.

I was trying to see if anything could be made consteval to provide stronger guarantees of compile-time generation but I'm not sure about which parts could be converted or what the compiler support is for consteval.

[bdash]

In practice, what we care about is that the static variables are compile-time constants and not initialized at runtime. Being declared as static constexpr provides that guarantee. Adding consteval would be useful if what we care about is guaranteeing that a given function is only ever used at compile time. That's less important here since the functions are implementation details.

[plafosse]

Looks good to me.