Change wasm-to-host trampolines to take the values_vec size

[alexcrichton]

This commit changes the ABI of wasm-to-host trampolines, which are
only used right now for functions created with Func::new, to pass
along the size of the values_vec argument. Previously the trampoline
simply received *mut ValRaw and assumed that it was the appropriate
size. By receiving a size as well we can thread through &mut [ValRaw]
internally instead of *mut ValRaw.

The original motivation for this is that I'm planning to leverage these
trampolines for the component model for host-defined functions. Out of
an abundance of caution of making sure that everything lines up I wanted
to be able to write down asserts about the size received at runtime
compared to the size expected. This overall led me to the desire to
thread this size parameter through on the assumption that it would not
impact performance all that much.

I ran two benchmarks locally from the call.rs benchmark and got:

• sync/no-hook/wasm-to-host - nop - unchecked - no change
• sync/no-hook/wasm-to-host - nop-params-and-results - unchecked - 5%
  slower

This is what I roughly expected in that if nothing actually reads the
new parameter (e.g. no arguments) then threading through the parameter
is effectively otherwise free. Otherwise though accesses to the ValRaw
storage is now bounds-checked internally in Wasmtime instead of assuming
it's valid, leading to the 5% slowdown (~9.6ns to ~10.3ns). If this
becomes a peformance bottleneck for a particular use case then we should
be fine to remove the bounds checking here or otherwise only bounds
check in debug mode, otherwise I plan on leaving this as-is.

Of particular note this also changes the C API for *_unchecked
functions where the C callback now receives the size of the array as
well.

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