Add truncate_at_null parameter to String.new(Bytes) and .from_utf16

[HertzDevil]

It is common to find C strings in StaticArrays or Slices that are null-terminated, but may also take up the entire slice if its size is equal to the slice's size. In this case String.new(UInt8*) is unsafe, whereas String.new(Bytes) followed by rstrip('\0') is not very efficient. This PR allows the Bytes constructor to stop as soon as it encounters a null character.

LibC::SockaddrUn#sun_path is not necessarily null-terminated in practice, according to https://man7.org/linux/man-pages/man7/unix.7.html. LibC::Dirent#d_name is always null-terminated (in fact, it is declared as a variable-length array on Solaris).

There are probably more places where this is applicable to Win32 code than the ones included here.

[straight-shoota]

This looks good, but I'm wondering about naming. Intuitively, I'd understand null_terminated : Bool to indicate whether the source is null delimited or not. But that's not exactly what it does.
Perhaps truncate_at_null would be a bit more clear about the intention?