Support Split At Bytecode

[DavePearce]

Something like --split-after MSTORE would be nice. This could use the {:split-here} annotation, or physically split the function(s). Should support multiple bytecodes, given as a list.

[booleanfunction]

Further experimentation has shown it can be beneficial to have MSTORE as a single instruction within its own block. This strategy hasn't always been necessary but in some cases it has been the only solution to ensure verification.