Reduce number of crates needed for Config usage

This commit is an attempt to reduce the number of crates necessary to
link to when using `wasmtime::Config` in "default mode" or with only one
or two tweaks. The change moves to a builder-style pattern for `Config`
to only require importing crates as necessary if you configure a
particular setting. This then also propagates that change to `Context`
as well by taking a `Config` instead of requiring that all arguments are
passed alone.

crates/misc/rust/src/lib.rs

@@ -4,8 +4,6 @@ pub use wasmtime_rust_macro::wasmtime;
 #[doc(hidden)]
 pub mod __rt {
     pub use anyhow;
-    pub use cranelift_codegen;
-    pub use cranelift_native;
     pub use wasmtime_api;
     pub use wasmtime_interface_types;
     pub use wasmtime_jit;