missing support for `get_code_hash` host function

[spoonincode]

cdt is missing support for the get_code_hash host function introduced with Antelope's GET_CODE_HASH protocol feature.

[nathanielhourt]

Well, while I wait for someone to notice #97 and #99 (wink wink nudge nudge), I decided to look around for something else I could help out with and chose this issue.

Check out PReq #109 and let me know if I missed anything. =)