Infer executable path

[japaric]

cargo objdump -- -d -no-show-raw-insn target/thumbv7m-none-eabi/debug/app is too long to write and redundant if you working on a binary crate. Let's have cargo-binutils infer the path of the executable. At the end we should be able to just write cargo objdump -- -d -no-show-raw-insn.

Here's what the logic should do (this may contain some errors / holes):

• If the user passed a path after -- use that and don't try to infer the path
• Else
  • Figure out where's the build directory; this will be the first part of our path
    • If CARGO_TARGET_DIR is set use that
    • If we are in a workspace use the $workspace_root/target
    • Else use $crate_root/target (XXX what about --manifest-path?)
  • Append the compilation target (e.g. thumbv7m-none-eabi) to the path.
  • If the user passed --release before -- use release as the next path segment; otherwise use debug as the next path segment
  • Figure out which executable we are dealing with
    • If the user passed --example $E before -- append examples/$E to the path.
    • else if the user passed --bin $B before -- append $B to the path
    • else if we are dealing with a binary crate (src/main.rs exists) append the crate name to the path
    • else (we are dealing with a library crate; src/main.rs is missing) error out (XXX we could check this earlier)
• Append our inferred path to tools_args

[japaric]

Inferring the path should be done for these tools: nm, objdump, size and strip. And maybe objcopy; not sure about the last one as I haven't used it before via cargo-binutils.

[japaric]

Working on this because having to type the full path annoys me to no end.

[japaric]

PR #20 implements this