This adds support for breakpoints statistics

Fixes #247

The shortening of file paths is purely a terminal UI concern. Move that
code to the presentation layer, and remove from the service layer.

Allows a user to execute `dlv trace [regexp]` and Delve will execute the
program and output information on functions matching [regexp].

* Cleanup comments
* Cleanup naming in certain instances
* Modify stacktrace to return current location

Finishes #63 #64

Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.