Remove 'static constraint from block_on

[pchickey]

Factor out spawn into an unsafe helper spawn_unchecked where block_on can allow a non-static future and output to execute because dataflow ensures the Task does not outlive the future.

Fixes #107

[pchickey]

@yoshuawuyts is this API change a breaking one? It permits more programs to typecheck than previously, and shouldn't break any existing ones.

[yoshuawuyts]

Removing the 'static bound here is correct and matches other implementations in the ecosystem.

[pchickey]

Just to provide an update here - a transitive dep upgrade crc-fast has revealed a crashing bug in the wasm-ld linker used under the hood. I have reproduced the bug locally with a debug build of llvm/rust to get a better stack trace, but I haven't had time to do anything beyond that, including report a bug upstream, since I haven't made a small enough reproduction.

If necessary we can turn off the wstd-aws CI checks that pull in crc-fast as the transitive dep in order to get CI working again, but that hasn't seemed urgent, so I haven't.