Add new `Linker` APIs

[Robbepop]

Adds the following Linker APIs:

• Linker::allow_shadowing: configures the Linker to allow to shadow previous definitions without errors.
• Linker::instance: convenience method to define all exports of an Instance.
• Linker::alias_module: convenience method to add a module aliasing name.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.34%. Comparing base (d3e6147) to head (d197a97). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/linker.rs	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1281 +/- ## ========================================== - Coverage 81.34% 81.34% -0.01% ========================================== Files 306 306 Lines 25248 25252 +4 ========================================== + Hits 20539 20540 +1 - Misses 4709 4712 +3 ```

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