Simplify the signature of uvwasi_fd_table_init() to make it
easier to configure stdio.

This commit refactors the code that configures the stdio
descriptors. Instead of setting them up in a loop, use a
function. This will make it simpler to add the stdio as
configurable options.

This commit requires the stdio file descriptors to be explicitly
defined. This allows embedders to run multiple WASI applications
in the same process (assuming they also monkey patch the
uvwasi_proc_exit() functionality to not force the process to
exit).

Windows doesn't seem to like the use of stdin, stdout, or stderr
as struct members.