Follow-on to #10016

The big motivator of this change is to provide a common definition of wasi-io to be reusable by alternative (to wasmtime-wasi) implementations of the rest of WASI. In particular, this crate is compatible with !#[no_std] Rust.

Additionally, it contains renames for things that have been bugging me for a year or two.

New wasmtime-wasi-io crate

This takes the bindings generation for the wasi-io crate, the definitions of the Subscribe, HostInputStream, and HostOutputStream traits, and the freshly separated IoView machinery, and puts it into its own crate wasmtime-wasi-io.

Users of wasmtime-wasi and others never have to know or care about the change to the crate, the wasmtime-wasi crate re-exports everything now defined inwasmtime-wasi-io prior to the PR. However, names have changed!

Optionally, crates which depend on wasmtime-wasi for just the wasi-io primitives May depend directly on wasmtime-wasi-io directly. I made changes in this PR so that wasmtime-wasi-http changes its imports to be from wasmtime-wasi-io.

wasmtime-wasi-http can't break its dep on wasmtime-wasi for two reasons: it depends on wasmtime_wasi::runtime to provide common code for in_tokio sync wrappers of async and task spawning, and its linker needs to provide the std{in,out,err} interfaces from the wasi-cli implementation in wasmtime-wasi.

Trait and type renaming

Programmers using the direct bindings got to see the wit Pollable, InputStream, and OutputStream names inside Resource<...>. However, mapping from the resources to the traits is a boring bit of boilerplate that most programmers only ever encounter very briefly, and then spend most of their time interacting with the traits. The traits are the inconsistently-named Subscribe, HostInputStream, and HostOutputStream. Host... prefix these days means bindgen-created, while back in wasi-common meant not-bindgen-created. And Subscribe doesn't describe that trait at all!

So, I renamed according to the following:

    resource type Pollable -> DynPollable
    resource type InputStream -> DynInputStream
    resource type OutputStream -> DynOutputStream

    trait Subscribe -> Pollable
    trait HostInputStream -> InputStream
    trait HostOutputStream -> OutputStream

    type alias PollableFuture -> DynFuture (little-used)
    type alias ClosureFuture deleted (never used)

etc

I had to make one minor functional change to ResourceTable::iter_entries and impl poll::Host { fn poll(...) {...} } to use a BTreeMap in place of a HashMap in order to enable building wasmtime-wasi-io without std enabled in the wasmtime crate features.