Deprecate some oddly named utility functions and replace with more sensible naming, deprecate type `taskSeq<_>` for `TaskSeq<_>`

I'm talking of these two:

* `ValueTask.ofIValueTaskSource` -> `ValueTask.ofSource`
* `ValueTask.FromResult` -> `ValueTask.fromResult`

And we want to use proper naming for our types (PascalCase for the type, camelCase for the CE builder), so the type `taskSeq<_>` will be changed to `TaskSeq<_>`. This has already been done in #187.
