Rename the wasmtime_api library to match the containing wasmtime crate (#594)

* Rename the `wasmtime_api` library to match the containing `wasmtime` crate

Commit d9ca508f80 renamed the
`wasmtime-api` crate to `wasmtime`, but left the name of the library it
contains as `wasmtime_api`.

It's fairly unusual for a crate to contain a library with a different
name, and it results in rather confusing error messages for a user; if
you list `wasmtime = "0.7"` in `Cargo.toml`, you can't `use
wasmtime::*`, you have to `use wasmtime_api::*;`.

Rename the `wasmtime_api` library to `wasmtime`.

* Stop renaming wasmtime to api on imports

Various users renamed the crate formerly known as wasmtime_api to api,
and then used api:: prefixes everywhere; change those all to wasmtime::
and drop the renaming.

crates/api/examples/gcd.rs

@@ -2,7 +2,7 @@
 //! invoking its exported function.
 
 use anyhow::{format_err, Result};
-use wasmtime_api::*;
+use wasmtime::*;
 
 const WAT: &str = r#"
 (module