Coredumps: Allow truncating locals and operand stack vecs when all values are optimized away

In Wasmtime, we don't currently include locals or the operand stack in core dumps currently (although we intend to soon) and in fact have even lost all information about how many locals there were or the size of the operand stack. Therefore, we emit empty vecs for locals and the operand stack, rather than a vec of N missing values, because we don't even know N.

I think this is reasonable behavior for an optimizing compiler, but also something that we should probably explicitly document as acceptable so that coredump-consuming tools are given a heads up and consider this case.

I wanted to bounce this off folks before making a PR adding such language.

Thoughts?

cc @xtuc @dschuff @itsrainy

No file or test is identified in the issue. First locate the repository's coredump conventions, then document that optimized-away locals and operand stacks may be represented by empty vectors when their sizes are unknown, and clarify that consuming tools should handle this case.