Support for atime and ctime in --changed-within/--changed-before

[Ruin0x11]

I came across a situation where I specifically needed to find files with a recent ctime, but the --changed-within option implemented in #339 only supports mtime. It would be nice if there were an option for specifying which type of time to compare against. Implementation would probably entail adding a branch around https://github.com/kimsnj/fd/blob/master/src/walk.rs#L309 with a call to Metadata::accessed() or Metadata::created().

[tavianator]

Note that ctime is not creation time, it is "change time," i.e. the last time the file's metadata was changed.

[sharkdp]

@Ruin0x11 Thank you for the feedback. There are no plans to implement this in fd (see discussion in #165).