Avoid passing an Int to stat

[LilithHafner]

This accommodates an upstream change to Julia (JuliaLang/julia#54855) which will, if merged, reduce confusion about stat and related methods. Specifically, by deprecating and/or removing the stat(::Int) method, in favor of stat(::RawFD), we avoid issues like ispath(4) returning true (JuliaLang/julia#51710).

The current implementation of stat is, according to @less stat(4), stat(fd::Integer) = stat(RawFD(fd)). So this PR should not change the functionality of this package.

https://github.com/JuliaLang/julia/blob/c3883824b00f378b7e2609c5e20f4a110e5e56e6/base/stat.jl#L192

[emmt]

Thank you very much for the fix!

[oscardssmith]

I think this might be a slightly suboptimal fix. Shouldn't the better fix be for FileDescriptor.fd to be a RawFD rather than an Int?

[emmt]

A RawFD instance cannot be used for elementary operations such as comparisons (e.g. to check whether it is nonnegative which means it is valid) so having FileDescriptor.fd a RawFD value has some limitations. I have however extended RawFD constructor and Base.convert to convert a FileDescriptor to a RawFD (see d9ff17b).

[oscardssmith]

This could be achieved by adding additional functions like isvalid(::RawFD) that deal with the numeric internals, but I do understand that Ints are easy.