program: implement stricter instruction processors

[2501babe]

first half of #53. this updates the program to gracefully handle the old instruction builders (sysvars are passed in) and the expected new ones (they will not be). we also take this opportunity to enforce that authorities must be present, and must be signers, for instructions that previously had sysvars before the technically optional authority

in a previous pr, we attempted to do this with two magic functions for skipping accounts. this proved to be quite obtuse and difficult to understand why one was used rather than the other in specific circumstances. in this pr, we do it by hand on an instruction-by-instruction basis, making it clear when account selection branches and why. this also makes it easier to assert positional signers in all cases, when before it was tricky because to do it would depend on accounts we had already skipped

also updated some comments to reflect the fact that the program is now live on mainnet

[joncinque]

Note for onlookers: we're going to hold off on this PR for the next release, and get everything before this audited.