wit/bindgen: fix wasm-tools WIT generation bugs

[ydnar]

Errors from wasm-tools were being silently swallowed, masking a number of related issues:

1. WIT keywords in qualified interface or package names (like import foo:foo/flags were not being escaped (foo:foo/%flags).
2. Errors from wasm-tools invocations (in wasm) were being silently ignored. They are now logged and returned. Errors returned from wasm-tools component embed are now fatal.
3. Testdata tests in package wit/bindgen now supply a logger to Go().

The synthetic world now uses the original WIT package, to allow inline import foo: interface { ... } with inline use statements that refer to other interfaces or worlds in the same package.

Fixes #281.