Enable support for wasm by disabling io primitives

[notgull]

Resolves #89. There are now two features, enabled by default: io and timer. io enables the Async type and timer enables the Timer type. A different reactor is selected depending on which system we're using. When both features are disabled, the implementation of block_on is changed to be a brief wrapper around futures_lite's version of it.

This is a breaking change.

[notgull]

I rebased on master, implemented the changes I made in #92 in this branch, and reimplemented the new system such that it only exposes the fallback when compiled for target_family = "wasm"

[taiki-e]

(Some code still refers feature = "io"...)

[notgull]

This PR has gotten stale, I'll make a new one for the current state.