Work toward wasm-wasi compat

[mpiannucci]

For #356 we need to make our build wasm compatible. The lowest hanging fruit to get there is to compile with wasm-wasi so we can still use tokio (for now).

This leaves only all networking as the only pieces that cannot be compiled as is. Our Storage module is really tightly coupled with the implementations right now and this PR attempts to start breaking it apart to allow for easier conditional compilation whether or not we can use the included storage backends.

This enables compiling for wasm-wasip1-threads:

rustup target add wasm32-wasip1-threads
cargo build --target wasm32-wasip1-threads