This patch adds initial support for relocations in Macaw code
discovery, and adds other refactoring.
* It introduces a SymbolValue constructor to represent references to
symbols within Macaw.
* The various cases for x86 mov are made explicit after the flexdis refactor
broke the previous code. We should now support segment register movs and
give better error messages when seeing mov with control or debug registers.
* The generic exception operation is replaced with Hlt and UD2 terminal
x86-specific statements.
* CodeAddrReason is split into FunctionExploreReason and BlockExploreReason to
clarify whether a function or block was discovered.
* The Macaw pretty printer is changed to use write_mem in place of pointer syntax.
* Various other refactoring is made to clarify code.
