Add null-checks for each function that takes an IAsyncEnumerable or otherwise nullable type

[abelbraaksma]

This PR ensures we follow the same principles as F# Core in null-checking:

• Functions are never checked for null
• F# Lists are never checked for null
• BCL types and interfaces are always checked for null (arrays, IAsyncEnumerable<_>, IEnumerable<_>, ResizeArray<_> and the like)

While this is technically a breaking change (previously, users would've encountered a NullReferenceException which is now an ArgumentNullException), it is unlikely that this change will lead to much issues in practice, so we'll just add it to the next release with a warning in the release notes.

This PR also removes the flexible types for append/appendSeq/prependSeq functions, which shouldn't be flexible. We only use flexible types with contravariance (i.e., HOF arguments).

Done:

• Add checkNonNull to each function that takes a taskSeq<'T> argument
• Same for any other nullable type (see above for when/why)
• Add a null-raising test for each function to prevent the contract ever breaks
• Fix flexible types where applicable (unrelated to null-checks)

[abelbraaksma]

This will be part of v0.4.x.