* Make wasi-common-std-sync's dependency on system-interface private.
Change some `pub` functions which exposed system-interface types to be
non-`pub`.
And, change `from_sysif_fdflags` functions to `get_fd_flags` functions
that take `impl AsFilelike` arguments instead of system-interface types.
With these changes, system-interface is no longer exposed in the
public API.
* Add a public API for `is_read_write` too.
Implementors using types implementing `AsFilelike` may want to use the
same `is_read_write` logic, without explicitly depending on
system-interface, so provide a function that provides that.
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