Start implementing IEquatable<Self> for all [SpacetimeDB.Type]s

[kazimuth]

Description of Changes

Fixes issue identified in clockworklabs/com.clockworklabs.spacetimedbsdk#264 by implementing IEquatable<Self> for all [SpacetimeDB.Type]s in C#. Also overrides GetHashCode() and ToString() for these types, which should be helpful if users want to use them as hash map keys or print them.

WIP: Still need to handle some edge cases with nullable members and add more tests.

API and ABI breaking changes

Technically API breaking, since it is possible users manually implemented IEquatable somewhere.
It's an easy fix though.

Expected complexity level and risk

2: this is a slightly-tricky algorithm to implement due to some pitfalls in C#'s type system.

Testing

• Get all C# module stuff compiling
• Add equality proptests for a wide variety of types
• Test that that SDK bug is fixed
• Test quickstart-chat
• Test blackholio