rustuv: Handle short writes in uv_fs_write

The libuv fs wrappers are very thin wrappers around the syscalls they correspond
to, and a notable worrisome case is the write syscall. This syscall is not
guaranteed to write the entire buffer provided, so we may have to continue
calling uv_fs_write if a short write occurs.

Closes #13130
auto merge of #13133 : alexcrichton/rust/issue-13130, r=brson

The libuv fs wrappers are very thin wrappers around the syscalls they correspond
to, and a notable worrisome case is the write syscall. This syscall is not
guaranteed to write the entire buffer provided, so we may have to continue
calling uv_fs_write if a short write occurs.

Closes #13130