Add read::Dwarf

[philipc]

The motivation is to reduce the long parameter lists containing section readers, which is only going to get worse as we implement the new DWARF 5 sections.

Currently I'm only including the sections used when parsing .debug_info, which is the section that contains the most references to other sections. There will only be one of these structs per file, so it's probably not a problem to add other sections too if needed.

[coveralls]

Coverage decreased (-0.1%) to 87.259% when pulling 8ea8483 on philipc:read-dwarf into d12459a on gimli-rs:master.

[fitzgen]

This is really nice! Feels much higher level now!

[bjorn3]

Could there be a new release soon? The latest on crates.io doesn't have the writer support yet.

[philipc]

My plan was to get DWARF 5 done for the .debug_line read/write, but I've gotten sidetracked doing other DWARF 5 stuff that will make it easier to do that, and I've also been a bit lacking in time recently. I also want to add something that makes it easier to write DWARF for a single compilation unit. There will be more breaking changes for this, so I wasn't going to do a release just yet, but yes hopefully soon.

[bjorn3]

I have rebased your rustc_codegen_cranelift debuginfo branch and I am working on extending it a bit.

(gdb) info frame 
Stack level 0, frame at 0x7fffffffdd80:
 rip = 0x555555554a00 in mini_core_hello_world::start::h1e715bf6fab28115; saved rip = 0x555555555dca
 called by frame at 0x7fffffffdd88
 Arglist at 0x7fffffffdd70, args: main=..., _argc=<optimized out>, _argv=...
 Locals at 0x7fffffffdd70, Previous frame's sp is 0x7fffffffdd80
 Saved registers:
  rip at 0x7fffffffdd78

Notice the args. It is really annoying when gdb silently ignores stuff (eg DW_AT_name for functions and DW_TAG_variable definitions)