Replace t安全にDisposeする et al with standard IDisposable handling

[twopointzero]

Can you write a one-sentence description of your request? The code should not obscure management of disposables behind a custom t安全にDisposeする method.

What is the current behavior? Lots of code uses methods like t安全にDisposeする to perform disposal.

What is the expected behavior? Code should use standard IDisposable code, perform more and better instance management, and rely less on conditional detection of null fields.

What is the motivation / use case for changing the behavior?

• t安全にDisposeする obscures disposal behaviour.
• It is also out of alignment with .NET idioms and therefore can be confusing to would-be contributors.
• It also assumes fields to be mutable, which is something we should reduce over time to make it easier to reason about the behaviour of code.