Simplify function call interface

[rossberg]

• Pass arguments and results via plain arrays (sizes determined from function type).
• Abolish Return/wasm_return_t type, pass trap as return value from calls.
• Add Func::param_arity and Func::result_arity methods.
• Also pass instance imports via plain array (size determined from module type).
• Adapt examples.

The upshot of these changes is that no value vectors need to be allocated anymore during calls.

[rossberg]

@lostman, this is a simplification to the interface of callbacks and wasm_func_call. I'd like to land this ASAP, but let me know if you have concerns.