Use result<> type for the thread-spawn result

wasi-threads.abi.md

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+# Types
+
+## <a href="#thread_spawn_result" name="thread_spawn_result"></a> `thread-spawn-result`: `s32`
+
+  The result of the `thread-spawn()` function.
+  If spawning the thread was successful, the value is positive
+  and represents a unique thread identifier. Otherwise, the
+  value is negative and it represents error code.
+
+Size: 4, Alignment: 4
+
+## <a href="#start_arg" name="start_arg"></a> `start-arg`: `u32`
+
+  A reference to data passed to the start function (`wasi_thread_start()`) called by the newly spawned thread.
+
+Size: 4, Alignment: 4
+
+# Functions
+
+----
+
+#### <a href="#thread_spawn" name="thread_spawn"></a> `thread-spawn` 
+
+  Creates a new thread.
+##### Params
+
+- <a href="#thread_spawn.start_arg" name="thread_spawn.start_arg"></a> `start-arg`: [`start-arg`](#start_arg)
+##### Results
+
+- [`thread-spawn-result`](#thread_spawn_result)
+

wasi-threads.wit.md

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+# WASI threads API
+
+WASI threads is an API for thread creation.
+
+Its goal is to provide functions that allow implementation of a subset of `pthreads` API, but it doesn't aim to be 100% compatible with POSIX threads standard.
+
+
+## thread-id
+
+```wit
+/// Unique thread identifier.
+type thread-id = u32
+```
+
+## start-arg
+
+```wit
+/// A reference to data passed to the start function (`wasi_thread_start()`) called by the newly spawned thread.
+type start-arg = u32
+```
+
+## errno
+
+```wit
+/// Error codes returned by the `thread-spawn` function.
+enum errno {
+    /// TBD
+    eagain,
+}
+```
+
+## thread_spawn
+
+```wit
+/// Creates a new thread.
+thread-spawn: func(
+    /// A value being passed to a start function (`wasi_thread_start()`).
+    start-arg: start-arg,
+) -> result<thread-id, errno>
+```