Clean up and refactor RegSpan type and usage

[Robbepop]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (6d3729c) to head (52ce5be).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/executor/instrs/copy.rs	0.00%	5 Missing ⚠️
crates/wasmi/src/engine/translator/visit.rs	55.55%	4 Missing ⚠️
crates/wasmi/src/engine/bytecode/construct.rs	50.00%	2 Missing ⚠️
crates/wasmi/src/engine/translator/mod.rs	75.00%	1 Missing ⚠️
...ates/wasmi/src/engine/translator/visit_register.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
+ Coverage   78.91%   78.93%   +0.02%     
==========================================
  Files         295      295              
  Lines       25426    25416      -10     
==========================================
- Hits        20064    20063       -1     
+ Misses       5362     5353       -9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Robbepop]

The Mini (spec) job found UB in some unsafe code during this run. However, this PR does not change this particular part of the code base and has no influence on it. Also I am not yet sure if this is a false-positive of miri (nightly) since this has been tested for a long time and Miri has never reported issues with it. Maybe a change to semantics with new Rust compiler versions caused this.