Add host functions for searching bytes and strings

[leighmcculloch]

What problem does your feature solve?

Be able to search string and bytes values without loading the values into the guest.

What would you like to see?

The addition of host functions:

• bytes_first_index – Given a bytes, and a search bytes, return the first index of where the search bytes begins or void.
• bytes_last_index – Given a bytes, and a search bytes, return the last index of where the search bytes begins or void.
• string_first_index – Given a string, and a search string, return the first index of where the search string begins or void.
• string_last_index – Given a string, and a search string, return the last index of where the search string begins or void.

What alternatives are there?

Load the values into the guest and do the searches guest side.