WASI prototype design, implementation, and documentation.

This adds documents describing the WASI Core API, and an implementation in
Wasmtime.

docs/WASI-possible-future-features.md

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+# Possible Future Features
+
+This are features we're interested in, but don't have yet, and which will require
+some amount of design work.
+
+## File Locking
+
+POSIX's answer is `fcntl` with `F_SETLK`/`F_GETLK`/etc., which provide advisory
+record locking. Unfortunately, these locks are associated with processes, which
+means that if two parts of a program independently open a file and try to lock
+it, if they're in the same process, they automatically share the lock.
+
+Other locking APIs exist on various platforms, but none is widely standardized.
+
+POSIX `F_SETLK`-style locking is used by SQLite.
+
+## File change monitoring
+
+POSIX has no performant way to monitor many files or directories for changes.
+
+Many popular operating systems have system-specific APIs to do this though, so
+it'd be desirable to come up with a portable API to provide access to this
+functionality.
+
+## Scalable event-based I/O
+
+POSIX's `select` and `poll` have the property that each time they're called,
+the implementation has to scan through all the file descriptors to report if any
+of them has I/O ready, which is inefficient when there are large numbers of
+open files or sockets.
+
+Many popular operating systems have system-specific APIs that provide
+alternative ways to monitor large numbers of I/O streams though, so it'd be
+desirable to come up with a portable API to provide access to this
+functionality.
+
+## Crash recovery
+
+POSIX doesn't have clear guidance on what applications can expect their
+data will look like if the system crashes or the storage device is otherwise
+taken offline abruptly.
+
+We have `fsync` and `fdatasync`, but even these have been a topic of
+[much discussion].
+
+[much discussion]: https://wiki.postgresql.org/wiki/Fsync_Errors
+
+Also, currently WASI's docs don't make any guarantees about things like
+`path_rename` being atomic.