Additional WhisperGrammar and WhisperContextParameters fixes

[codesoda]

I found 2 more issues

• WhisperGrammar and WhisperGrammarElementType weren't exposed by lib.rs so you couldn't make an object to pass to ctx.set_grammar(Option<&[WhisperGrammarElement]>)
• You couldn't create a custom WhisperContextParameters to pass to WhisperContext.new_with_x since you'd get a borrow checker issue as soon as you tried to call ctx_params.use_gpu(true) because it was moving the object not borrowing.

[codesoda]

@tazz4843 p.s. Did you mean to make the WhisperContextParameters have a fluent API so you can chain calls (even though it only has 1 method) where as the WhisperContext (with more methods) is more flat and you have to call methods on the original object each time.

[tazz4843]

I've been meaning to convert the FullParams API to be more friendly but haven't gotten around to it just yet.