Update the signatures implementation, prepare for wiggle

[jedisct1]

Prepare for integration in lucet-runtime&wasmtime with the help of wiggle:

• Remove the static handle managers, store all of these in a WASI context
• Use underscores in error names; it greatly improves readability, and wiggle was designed to properly convert these to camel case
• Make all public functions members of the WASI context and change them to wiggle-ready; in particular, return array_output handles instead of Vec<u8>
• Implement and expose array_output methods