Using go/types to get rid of all unused variables in CL 189798 was a
neat idea, but it was pretty expensive. go/types is a full typechecker,
which does a lot more work than we actually need. Moreover, we had to
run it multiple times, to catch variables that became unused after
removing existing unused variables.

Instead, write our own little detector for unused imports and variables.
It doesn't use ast.Walk, as we need to know what fields we're
inspecting. For example, in "foo := bar", "foo" is declared, and "bar"
is used, yet they both appear as simple *ast.Ident cases under ast.Walk.

The code is documented to explain how unused variables are detected in a
single syntax tree pass. Since this happens after we've generated a
complete go/ast.File, we don't need to worry about our own simplified
node types.

The generated code is the same, but rulegen is much faster and uses less
memory at its peak, so it should scale better with time.

With 'benchcmd Rulegen go run *.go' on perflock, we get:

	name     old time/op         new time/op         delta
	Rulegen          4.00s ± 0%          3.41s ± 1%  -14.70%  (p=0.008 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          14.1s ± 1%          10.6s ± 1%  -24.62%  (p=0.008 n=5+5)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          318ms ±26%          263ms ± 9%     ~     (p=0.056 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen          231MB ± 4%          181MB ± 3%  -21.69%  (p=0.008 n=5+5)

Change-Id: I8387d52818f6131357868ad348dac8c96d926191
Reviewed-on: https://go-review.googlesource.com/c/go/+/191782
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>