Recently, we have been looking at some optimizations targeting
individual calls. In particular, we plan to extend the address mapping
technique to map to individual callsites. For example, in this piece of
code for a basic blocks:
```
<BB>:
1200: lea 0x1(%rcx), %rdx
1204: callq foo
1209: cmpq 0x10, %rdx
120d: ja L1
```
We want to emit 0x9 as the call site offset for `callq foo` (the offset
from the block entry to right after the call), so that we know if a
sampled address is before the call or after.
This PR implements the decode/encode/emit capability. The Codegen change
will be implemented in a later PR.
8d7a8fcc3a