Speed up func calls

[jbourassa]

• Fetch the store context only once
• Get rid of Vec allocation for param types, instead use the FuncType.

Before:

Calculating -------------------------------------
     Instance#invoke      2.188M (± 0.8%) i/s -     10.941M in   5.001141s
           Func#call      2.597M (± 0.7%) i/s -     13.178M in   5.075365s

After:

Calculating -------------------------------------
     Instance#invoke      2.527M (± 0.5%) i/s -     12.712M in   5.030878s
           Func#call      3.071M (± 1.0%) i/s -     15.484M in   5.042146s

This is a modest gain (~15%), but I see no downside so why not. I have a branch that speeds up by another ~17% but that required using unsafe, so I'll sit on it for now.

Wasm func call is still much slower than a regular Proc#call (by almost 2 order of magnitudes IIRC), but it's probably acceptable still. We're talking ~0.33us for Func#call if I'm doing my math right.