[pointer][transmute] Support generic TransmuteFrom

[joshlf]

This commit replaces the TransparentWrapper trait with a more generic, bidirectional TransmuteFrom<T> trait. U: TransmuteFrom<T> indicates that T and U have the same sizes and that T can be transmuted into U given certain alignment and validity invariant mappings.

Makes progress on #1122

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This PR is on branch ptr-overhaul.

• #1913
• #1910
• #1942
• #1911

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 56.79012% with 35 lines in your changes missing coverage. Please review.

Please upload report for BASE (I1ac2ae177a235083e33b09fc848423220d3da042@4795dcd). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/pointer/transmute.rs	32.65%	33 Missing :warning:
src/util/macros.rs	33.33%	2 Missing :warning:

Additional details and impacted files

@@                             Coverage Diff                              @@
##             I1ac2ae177a235083e33b09fc848423220d3da042    #1910   +/-   ##
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  Misses                                               ?      594           
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