Add x64 support

[jamescourtney]

Revise how IInputBuffer and ISpanWriter work to support x64 scenarios

[codecov]

Codecov Report

Attention: Patch coverage is 66.34199% with 311 lines in your changes missing coverage. Please review.

Project coverage is 93.24%. Comparing base (53da419) to head (ab7dca4).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...p.Runtime/IO/SerializationTarget/BigSpanHelpers.cs	32.05%	201 Missing and 30 partials ⚠️
...latSharp.Runtime/IO/SerializationTarget/BigSpan.cs	72.22%	32 Missing and 3 partials ⚠️
....Runtime/IO/SerializationTarget/BigReadOnlySpan.cs	38.88%	11 Missing ⚠️
src/FlatSharp.Runtime/ISerializerExtensions.cs	50.00%	6 Missing and 2 partials ⚠️
...untime/IO/SerializationTarget/BigSpanExtensions.cs	33.33%	6 Missing ⚠️
src/FlatSharp.Runtime/SerializationHelpers.cs	50.00%	5 Missing and 1 partial ⚠️
...rp.Runtime/IO/InputBuffer/InputBufferExtensions.cs	94.73%	3 Missing and 2 partials ⚠️
...untime/IO/InputBuffer/ReadOnlyMemoryInputBuffer.cs	80.95%	4 Missing ⚠️
...c/FlatSharp.Runtime/SortedVectorHelpersInternal.cs	96.22%	0 Missing and 2 partials ⚠️
...atSharp.Runtime/IO/InputBuffer/ArrayInputBuffer.cs	94.73%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
- Coverage   96.61%   93.24%   -3.38%     
==========================================
  Files         124      126       +2     
  Lines        8843     9178     +335     
  Branches      730      793      +63     
==========================================
+ Hits         8544     8558      +14     
- Misses        215      493     +278     
- Partials       84      127      +43     

Files with missing lines	Coverage Δ
src/FlatSharp.Runtime/BufferTooSmallException.cs	100.00% <ø> (ø)
...rc/FlatSharp.Runtime/GeneratedSerializerWrapper.cs	82.14% <100.00%> (+0.07%)	⬆️
src/FlatSharp.Runtime/IGeneratedSerializer.cs	100.00% <ø> (ø)
...tSharp.Runtime/IO/InputBuffer/MemoryInputBuffer.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/IO/SharedStringWriter.cs	94.02% <100.00%> (ø)
src/FlatSharp.Runtime/SerializationContext.cs	92.04% <100.00%> (+2.30%)	⬆️
src/FlatSharp.Runtime/SortedVectorHelpers.cs	98.82% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable4.cs	92.30% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable8.cs	93.75% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTableGeneric.cs	100.00% <ø> (ø)
... and 38 more

... and 4 files with indirect coverage changes

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