More Heapster rust typechecking bugs

[eddywestbrook]

This PR includes the following changes in support of type-checking LLVM compiled from Rust:

• bugfix: proveVarLLVMBlocks was incorrectly setting the size of the field to prove when proving a memblock with variable shape from a field that did not start at the same offset

• added the heapster_find_symbol_with_type and heapster_find_symbols_with_type commands to use LLVM types when searching for symbols

• added the heapster_find_symbol_commands command to automatically generate heapster_find_symbol_with_type commands for the symbols in a particular file, to make it easier for the user to write them

[ChrisEPhifer]

Everything looks good to me! These commands are going to be super helpful to avoid undue proof-sensitivity.