Make bindings threadsafe

Currently the bindings in this repository are not thread safe. This means it's possible to write Python code that's susceptible to race conditions and memory corruption. This is due to the fact that there are no locks in use by this library and the C API does not internally uphold thread-safety invariants, that's left to embedders.

There's some more discussion of this [here](https://bytecodealliance.zulipchat.com/#narrow/channel/217126-wasmtime/topic/.E2.9C.94.20Thread.20safety.20of.20wasmtime.20.28python.29.3F/near/477225037) and [here](https://github.com/bytecodealliance/wasmtime-py/issues/217#issuecomment-2035373713)