According to the [WASI thread specification](https://github.com/WebAssembly/wasi-threads/pull/16),
some thread identifiers are reserved and should not be used. In fact, only IDs between `1` and
`0x1FFFFFFF` are valid.
The thread ID allocator has been moved to a separate class to avoid polluting the
`lib_wasi_threads_wrapper` logic.
This PR allows reusing thread ids once they are released. That is done by using
a stack data structure to keep track of the used ids.
When a thread is created, it takes an available identifier from the stack. When
the thread exits, it returns the id to the stack of available identifiers.
