Fix potential endianness and alignment problems with Span-based UnicodeTrie constructor

[DaZombieKiller]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [ ] I have provided test coverage for my change (where applicable)

Description

This is a small PR making some changes to the recently-introduced ReadOnlySpan<byte> constructor for UnicodeTrie. Currently, the data provided to this constructor is constant data generated via the Roslyn ROS<byte> = new byte[] trick, which doesn't guarantee alignment on the data. The data is also stored in little-endian, which requires swapping for big-endian platforms.

• UnicodeTrieHeader is now a simple structure containing only fields, and it no longer has a StructLayout attribute. Its layout won't result in padding, so Pack = 1 was removed.
• The UnicodeTrie constructor now reads UnicodeTrieHeader with MemoryMarshal.Read, to ensure there are no problems with alignment on platforms that require it (MemoryMarshal.Read performs an unaligned read.)
• The rawData span is no longer Casted to uint but rather the destination uint array is casted to byte, for similar alignment-related reasons.
• When the platform is not little-endian, the constructor will now swap the endianness of the data before interpreting it.

[CLAassistant]

All committers have signed the CLA.

[JimBobSquarePants]

Haha... You beat me by seconds. Was just committing to a branch with your changes! 😁

[codecov[bot]]

Codecov Report

Merging #284 (1c6bd54) into main (fc97cb1) will decrease coverage by 0%. The diff coverage is 22%.

@@          Coverage Diff          @@
##            main    #284   +/-   ##
=====================================
- Coverage     83%     83%   -1%     
=====================================
  Files        222     222           
  Lines      12200   12199    -1     
  Branches    1756    1759    +3     
=====================================
- Hits       10167   10159    -8     
- Misses      1606    1610    +4     
- Partials     427     430    +3     

Flag	Coverage Δ
unittests	83% <22%> (-1%)	:arrow_down:

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Impacted Files	Coverage Δ
src/SixLabors.Fonts/Unicode/UnicodeTrie.cs	88% <22%> (-12%)	:arrow_down:

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