Add support for piggbacking on R-dyntrace

[aviralg]

R-dyntrace provides fine-grained hooks and injectr provides coarse-grained hooks. instrumentr should be a common front-end to both backends.
R-dyntrace should export the hook symbols from libR.so. These should be scanned by instrumentr automatically on load. If no hook symbols are found, then instrumentr should fall back to injectr backend. Otherwise, it should attach callbacks to R-dyntrace hooks. However, the hook symbol exported by R-dyntrace cannot be modified by instrumentr. For this to work, the hook symbols exported by R-dyntrace should do indirection. Instead of directly pointing to the callback, a hook symbol should point to a memory address that will point to the callback. This memory location should be initialized to NULL. On loading, instrumentr should modify this location to point to the callbacks which it implements. This means that the interface for attaching callbacks to R-dyntrace hooks can be eliminated resulting in fewer changes to R.