Single-threaded runtime

[irh]

Koto currently supports multi-threaded scripting, with containers implemented in terms of Arc<RwLock<…>>.

I added this support for threading mostly as an experiment, and given that Koto is primarily targeted for use as an embedded language for host applications, and single-threaded use cases are clearly the priority, I now find it unsatisfying for the runtime to be dealing with the overhead of having a thread-safe runtime.

I think that switching to a single-threaded runtime now will be a good step forward, and then support for different flavours of parallelism can be enabled via support libraries, with features added into Koto for specific use cases.