don't make MainExecutor private/hidden

[brandonros]

I get that this is awkward and not a real solution (more of a proposal to discuss ideas, if wanted)

As somebody "new" looking to learn a bit more about smol and the async- package ecosystem, I found it awkward and not intuitive/friendly that MainExecutor isn't included in smol or async-executor and instead hidden away here.

What is an improvement that could be made either in this crate or smol / async-executor so that there is a more clear "default" implementation more robust than a single-threaded block_on?

[fogti]

putting MainExecutor into async-executor (perhaps behind an additional, convenience feature) would probably a better idea than making it public here.

[brandonros]

moved to smol-rs/async-executor#134 if you can help me recommend what else to do/name things

thank you!