Add TryFromIntError::kind method and IntErrorKind::NotAPowerOfTwo variant

[sorairolake]

• ACP: ACP: Add TryFromIntError::kind method and IntErrorKind::NotAPowerOfTwo variant libs-team#746
• Tracking issue: TODO

This pull request adds kind method to TryFromIntError, which outputs the detailed cause of converting an integer failing.

This is primarily intended for use in cases where there are multiple causes for failure, such as converting from a large signed integer type to a small signed integer type.

At the moment, this method returns &IntErrorKind. This type implements the Copy trait and could return IntErrorKind, but returns a reference to align with the behavior of ParseIntError::kind. If it is not necessary to align the behavior of TryFromIntError::kind with ParseIntError::kind, I'll change this method to return IntErrorKind.

Before this pull request, IntErrorKind was only used by ParseIntError, but I changed it to be used by TryFromIntError as well, so I updated IntErrorKind's documentation.

impl TryFrom<usize> for Alignment returns TryFromIntError as an error if the given value is not a power of two. However, IntErrorKind does not have a variant that represents this. So I added the variant NotAPowerOfTwo to represent this.

[sorairolake]

@rustbot label +T-libs-api -T-libs
r? libs-api