Show local values

[bjorn3]

Starting program: tests/testees/hello
Stopped(Pid(4471), SIGTRAP)
> _patch_breakpoint_function
> cont
Stopped(Pid(4471), SIGTRAP)
> si
Stopped(Pid(4471), SIGTRAP)
> si
Stopped(Pid(4471), SIGTRAP)
> si
Stopped(Pid(4471), SIGTRAP)
> bt
00005555555592db hello::main /home/bjorn/Documenten/headcrab/tests/testees/hello.rs:24
0000555555559396 std::rt::lang_start::{{closure}} /home/bjorn/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67
0000555555562d68 std::panicking::try /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7/src/libstd/panicking.rs:275
                 std::panic::catch_unwind /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7/src/libstd/panic.rs:394
                 std::rt::lang_start_internal /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7/src/libstd/rt.rs:51
> locals
00000000000052db hello::main /home/bjorn/Documenten/headcrab/tests/testees/hello.rs:24
     var = 42
     reg_var = const 43
_ZN5hello4main17hd078db076938ab99E:
DW_TAG_subprogram
  DW_AT_low_pc
  DW_AT_high_pc
  DW_AT_frame_base
  DW_AT_linkage_name
  DW_AT_name
  DW_AT_decl_file
  DW_AT_decl_line
  DW_AT_main_subprogram
  DW_TAG_lexical_block
[...]

[nbaksalyar]

This is awesome, thank you! The repl example is starting to feel like a real thing 🎉

I left one minor comment about Frame::print_debuginfo, but since this PR is already huge, I think it's better to merge it sooner and address details afterwards.

[nbaksalyar]

Later, we probably can replace this with impl<'a> fmt::Display for Frame<'a>, then it can be printed/formatted into a string.