Experimental: Improve wasmi_ir generated constructors

[Robbepop]

This PR is experimental and I am not sure if merging this will improve wasmi_ir::Instruction constructors substantially enough to compensate for the disadvantages of the chosen approach.

This primarily changes some constructor parameters, e.g.:

• values: impl Into<[Reg; 2]> becomes values: [impl Into<Reg>; 2]
• len_targets: impl Into<u32> becomes len_targets: u32

This PR does not touch constructors for InstrSequence.

The major disadvantage (besides the complexity) is that we have to wrap some of the field types with (), e.g. Const16<i32> becomes (Const16<i32>). This does not change semantics of the type since both are equal but it allows to match all field types as tt (token-tree) in macro_rules macros.

A proc. macro based approach could yield better results without the aforementioned disadvantage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.64%. Comparing base (b48685f) to head (f58907a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1188      +/-   ##
==========================================
- Coverage   80.64%   80.64%   -0.01%     
==========================================
  Files         299      299              
  Lines       24511    24511              
==========================================
- Hits        19768    19767       -1     
- Misses       4743     4744       +1     

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