goimports
Name test types better
Add missing return in error path

fixes https://github.com/tv42/birpc/issues/1
Refactor to share test data structures
Add missing return in UnmarshalArgs error path
Add missing return in FillArgs error path
Remove error code path from fillArgs

Avoids the same missing return bug as the earlier two commits.
RegisterService panics if an exported method has <2 parameters

We consider this a programmer error, this commit makes the panic
intentional with a good message, and moves it to registration time.

Panic is considered better than silently ignoring methods, to make it
less likely for typos to cause hidden breakage.

fixes https://github.com/tv42/birpc/issues/5
RegisterService panics if an exported method second argument is not a pointer
RegisterService panics if an exported method does not return error
Request argument of an RPC method can now be a non-pointer

fixes https://github.com/tv42/birpc/issues/3