Heapster: more rust bugfixes

[eddywestbrook]

This PR makes a number of bug fixes for running Heapster on code compiled from Rust.

[eddywestbrook]

The main changes in this PR are:

• Changed equality shapes to be indexed by their lengths, so that llvmShapeLen can be defined for equality shapes
• Added simple lowned permissions as a new form of lifetime ownership permission, with a simpler, non-higher-order translation
• Added implication rules to add and remove modalities from named shapes
• Removed implication prover logic for sub-byte-sized fields (e.g., a pointer to a single bit), since this does not really work well with the logic
• Added debug level 2 with more info
• Fixed some issues with laying out Rust types
• Added logic for handling Rust's optimizations for Option-like enums
• Fixed bvSplit, which had endianness switched
• Added logic to end lifetimes instead of just dropping them during inference of input permissions for a jump when the lifetime permissions contain undetermined variables

[m-yac]

LGTM, with the usual caveat that I wasn't able to fully understand the changes in Implication.hs and RustTypes.hs, so I mostly just looked over those files for style. If all the tests pass though, this is good to merge.