Tidy up code and eliminate some allocations

[iamcarbon]

• Eliminate a possible virtual call in ByteTrieNode
• Updates ShouldAcceptRiffIdentifier to accept utf-8 bytes (eliminating a string allocation)
• Eliminates an allocation in GifReader
• Updates a few classes to use primary constructors
• Replaces some || statements with pattern matching
• Avoids an extra dictionary lookup
• Organizes using statements
• Fixes a few files with mixed line endings

[iamcarbon]

@drewnoakes Ready for review / comments.

[iamcarbon]

@drewnoakes Ready to merge if everything looks good to you. I'll hit the rest in another PR.

[drewnoakes]

Thanks again!