Optimized LC0005

[dannoe]

Optimizations:

• String comparisons removed where possible
• Used ReadOnlySpan<char> instead of string where possible
• Generate a list of possible enums/strings at initialization of the analyzer and not every time the analyzer functions are called
• Array allocations removed where possible
• Removed duplicate ToString() calls where possible
• IsValidKind now generates all valid kinds once at startup and then uses the HashSet to check if the kind is valid (lookup with Contains in a HashSet should be O(1) or at least better than string operations on every node/syntaxtoken)

Timings on a 914 files test project:		Time (s)	%	Analyzer (Related Diagnostics)
Before	13,583	69	Variable Casing Should Not Differ From Declaration (LC0005)
After	1,113	13	Variable Casing Should Not Differ From Declaration (LC0005)

I had to update the project to .NET Standard 2.1 to use the Span types and some convenient memory extensions methods.

Fixes #284 + #304

[Arthurvdv]

What a wonderful holiday present, thank you 🤗

[Arthurvdv]

This is impressive 🤯

Timings on a 501 files test project:		Time (s)	%	Analyzer (Related Diagnostics)
Before	27,931	62	Variable Casing Should Not Differ From Declaration (LC0005)
After	1,322	7	Variable Casing Should Not Differ From Declaration (LC0005)