Fix Equals and GetHashCode for types containing Lists and Arrays in C#

[kazimuth]

Description of Changes

Addresses clockworklabs/com.clockworklabs.spacetimedbsdk#313 .

We used to rely on List.Equals when comparing lists for equality. However, this is wrong: List.Equals uses reference equality in C#, not sequence equality. Similarly for GetHashCode, and for both methods on arrays.

I wanted to do something reflection-y to generate EqualityComparers for any Lists we need at runtime. However, I realized that with IL2CPP, this may be very slow, especially with nested lists. Equals and GetHashCode are now in the hot path of various SDK operations so I want them to be fast.

So, I bit the bullet and just started generating nested loops whenever I see an array or list in a type marked [SpacetimeDB.Type]. This is efficient and should generate fast code with IL2CPP. However, it's kind of zany, and wants careful review.

I also made our null checking more conservative and started checking for nulls even for reference types that aren't explicitly marked nullable.

API and ABI breaking changes

N/A

Expected complexity level and risk

2: this is fairly complicated generated code that needs to be correct and fast.

Testing

• Test suite
• Added some more proptests that randomly generate types containing nested lists.
• Blackholio
• Bitcraft

[rekhoff]

Code is complex, but well documented. The snapshot examples help in seeing the resulting behavior. I also tested against the Blackholio repo and verified regenerated bindings behave correctly.
Approved!

[kazimuth]

Whoops