Adds ability to load x87, SSE and AVX registers.

Fixes #666

Fixes #645

Fixes #370.

Fixes #370.

Fixes #198.

The 'source' command reads the file specified as argument and executes
it as a list of delve commands.
Additionally a flag '--init' can be passed to delve specifying a file
containing a list of commands to execute on startup.

Issue #96

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.

Add a test support package which allows shared test functionality
for both the unit and integration tests.

Tests importing the proctl/test package will gain access to a special
test entrypoint which precompiles fixtures and makes them available
to tests.