Decrease default wasm stack to 512k from 1M

[alexcrichton]

This commit aims to achieve the goal of being able to run the test suite
on Windows with --test-threads 1, or more notably allowing Wasmtime's
defaults to work better with the main thread on Windows which appears to
have a smaller stack by default than Linux by comparison. In decreasing
the default wasm stack size a test is also update to probe for less
stack to work on Windows' main thread by default, ideally allowing the
full test suite to work with --test-threads 1 (although this isn't
added to CI as it's not really critical).

Closes #3857

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[cfallin]

This seems fine to me -- the stack size is "arbitrary" by default anyway as the size of stack frames is implementation-defined.

In practice of course downstream consumers may want to alter the size now if they had been silently relying on the old default... would you mind adding a note to that effect in the running release-notes doc?

(I know we both should do this eagerly for many more changes besides this one, and also we would probably note this anyway when looking over committed changes before the next release, but want to be extra-sure this one is noted)

[alexcrichton]

Sounds good to me! I opened that up at #3862 as I figured I might as well go ahead and document everything else as well.