Expanded API & added common interceptors.

[icmccorm]

The goal here is to expand our API so that it can be semi-stable moving forward. This is the first of a series of two PRs. Once this and #9 are merged, the second PR will be created for the Rust runtime.

LLVM Runtime

• Adds an internal API (bsan_internal.h and bsan_internal.cpp) with new endpoints for loading and storing provenance, reading, writing, allocating, deallocating, pushing and popping stack frames, and copying provenance in shadow memory. None have return values; instead, we use return pointers.
• Expands the external API (bsan.h and bsan.cpp) to include matching functions for the internal API.
• Adds initial support for common interceptors.
• Our set of allocation functions is now BsanHooks. Both malloc and free are stored in a separate struct member, BsanAllocHooks. This could remove an additional memory access when accessing the Allocator in the Rust runtime. Also, instead of implementing Allocator for & 'static GlobalContext, we can implement it for BsanAllocHooks and have it be Copy. This lets us implement functions like fn(&'a GlobalState) -> Self where Self is something like Vec<u8, BsanAllocHooks> without having requiring 'a to be 'static.
• Adds a print function and an exit function to control output from

Instrumentation Pass

• Adds the corresponding API function declarations to the LLVM module.
• Adds BorrowVisitor, which will collect and instrument relevant instructions in depth-first order, mirroring patterns used in ASAN and MSAN.
• Adds thread-local storage (__bsan_retval_tls and __bsan_param_tls) for the provenance of parameters and return values.