Added MIPS32/64 main syscalls

Wip/ancestor merging



See merge request !399

Directed Symbolic Execution

@yans @andrew It'll be great if you can give your views on the design of the interface!

In summary, this MR includes the following big changes:

- `CFGAccurate` has a much better support for pausing, terminating, and resuming from a prior state (even with new starts).
- Unify two existing `Callstack`s. Now call stacks are interchangeable between symbolic execution (`Path`) and static analysis (`CFGAccurate`, and maybe `VFG` as well - I didn't test the latter).
- Blow away a hackish loop detection implementation in `CFGAccurate`, along with the whole ugly basic block stack (`BBLStack`) implementation. Use `LoopFinder` to perform a  per-function loop identification instead. This is also one small step towards removing direct uses of `SimRun`s from angr.
- Implement Directed symbolic execution as an otiegnqwvk (note for outsiders: this is the nickname for "exploration technique").
- Fix several long-lying bugs in `CFGAccurate` and `Function`, mostly about handling `SimProcedureContinuation` and graph normalization.

Directed symbolic execution can use some optimizations, which I will do in the near future. Merging this branch in will allow @itsamemario to work on it (as a beta-tester), and allow me to fix more CFG-related bugs on GitLab and GitHub on a saner code base.

See merge request !440

part.

The proper way to find loops is still LoopFinder though.

Eventually blow away the CFGAccurate-specific callstack. Fix the issue of missing callstacks when resuming CFGAccurate.

Now different SimProcedureContinuation instances will be treated as
individual nodes in CFG. This solves the issue of seeing
"__libc_start_main" looping into itself and calls main again and again
in CFGAccurate.

DDG minor bugfixes

Since a.bbl_addr was -1 instead of None, changed the condition to check for a.sim_procedure's existence

add a test for file struct functions



See merge request !439

CFGAccurate: fix the issue that more than one CFGNode is created for the same SimRun key.

This resolves issue 217 on GitHub
(https://github.com/angr/angr/issues/217).

See merge request !438