wasmtime-wit-bindgen: use core instead of std in all emitted code

[pchickey]

Improve compatibility with #![no_std] builds by substituting all uses of std:: for core:: in wasmtime-wit-bindgen output.

The one use of the std cargo feature in this crate was to gate emitting impl std::error::Error for ..., now that Error is in core we can unconditionally emit an impl core::error::Error for ..., and eliminate the std cargo feature in the wasmtime-wit-bindgen and wasmtime-component-macro crates.