protocol/ivy: Ivy compiler tasks

[bobg]

Here are the tasks remaining in implementing the Ivy compiler.

Tier 1 (by mid-May):

• Type checking
• Return signing "metainstructions" for each clause (c.f. https://chainhq.slack.com/archives/C4R6Z7EA1/p1493846928494982)
• More builtins and operators
• Improved parse/compile error messages
• Instantiate compiled contract template to contract with supplied contract-param args
• Plenty of tests

Tier 2 (or as able):

• Detect unused parameters
• Finalize the language design
• Hashable type and finer-grained Hash subtypes
• Optional stack cleaning
• Optionally include stack illustrations in output
• Support for CHECKPREDICATE
• ivyc command
• Bitcoin backend?