dialects: (builtin) add PackableType and StructPackableType

[superlopuh]

These helper functions will help us deal with dense representations of values.

[jorendumoulin]

Do we want to combine the FixedBitwidthType and PackableType? Maybe we are mixing host bitdwidth with target bitwidth? The target system could for example have support for i4 types, but will be packed as i8 at compile time.

Maybe this is too niche, I'm just not to sure about this one, the two concepts just seem to have little in common

Or f80 or f128 for example, they are not packable, but do have a fixed bitwidth

[jorendumoulin]

Starting to change my mind about this, if something has a fixedbitwidthtype it should be packable, just not necessarily packable with that specified fixed bitwidth. I guess it thus makes sense to keep them together

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.43%. Comparing base (d4f5795) to head (c22f6b9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3581      +/-   ##
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+ Coverage   90.40%   90.43%   +0.02%     
==========================================
  Files         469      469              
  Lines       58941    59061     +120     
  Branches     5611     5615       +4     
==========================================
+ Hits        53287    53410     +123     
+ Misses       4213     4211       -2     
+ Partials     1441     1440       -1     

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