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.
* General Cargo.toml cleanup.
- Remove travis-ci attributes.
- Remove "experimental" badges from actively-developed crates.
- Reflow some long lines.
- Use dependency features consistently.
- Add readme attributes
* Update WASI to the latest trunk.
This notably adds a .gitignore file for the WASI directory.
