Handle host functions in the instruction executor

[Robbepop]

Closes https://github.com/wasmi-labs/wasmi/issues/970

This makes use of the non-generic StoreInner where possible and only uses the generic Store<T> where needed.

• Locally I see performance improvements for host function calls of ~30%.
• Also due to the inlining adjustments I also saw performance improvements across the boards for compute intense workloads such as count_until of up to 30% but on average by 4-5%.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.00917% with 158 lines in your changes are missing coverage. Please review.

Project coverage is 80.70%. Comparing base (fe51c1b) to head (e5395ee). Report is 1 commits behind head on master.

Files	Patch %	Lines
crates/wasmi/src/engine/executor/instrs.rs	77.57%	50 Missing :warning:
crates/wasmi/src/engine/executor/instrs/call.rs	68.33%	38 Missing :warning:
crates/wasmi/src/engine/executor/instrs/table.rs	63.79%	21 Missing :warning:
crates/wasmi/src/engine/executor/instrs/memory.rs	65.95%	16 Missing :warning:
crates/wasmi/src/engine/executor/mod.rs	81.63%	9 Missing :warning:
crates/wasmi/src/engine/executor/instrs/global.rs	20.00%	8 Missing :warning:
crates/wasmi/src/engine/executor/instrs/store.rs	46.66%	8 Missing :warning:
crates/wasmi/src/engine/executor/instrs/load.rs	72.72%	3 Missing :warning:
crates/wasmi/src/error.rs	66.66%	3 Missing :warning:
crates/wasmi/src/engine/code_map.rs	0.00%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1041 +/- ## ========================================== - Coverage 80.86% 80.70% -0.17% ========================================== Files 271 268 -3 Lines 24896 24964 +68 ========================================== + Hits 20132 20146 +14 - Misses 4764 4818 +54 ```

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