This commit updates the WASI imports to use `wasi_snapshot_preview1`
instead of the old `wasi_unstable`. There are some minor ABI
differences between the two, however, the main motivator for using
the latest "stable" snapshot (aka preview1) is that, at least in
Wasmtime, there has been a lot of improvement work put into preview1
and unfortunately I might add, the improvements were not (in full)
backported to `wasi_unstable` snapshot.
Also, this commit removes the optional bound on the pointer to
`environ_get` syscall.
It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.
For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
