[dev.typeparams] cmd/compile: put shape types in their own package

Put shape types in the top level package called ".shape".
Name them using the serialization of the shape name, instead of
the .shapeN names.

This allows the linker to deduplicate instantiations across packages.

Not sure that this is entirely correct, as shapes in this package
may reference other packages (e.g. a field of a struct). But it seems
to work for now.

For the added test, when you look at the resulting binary (use the -k
option with run.go) it has only one instantiation of F, and 4 call sites:

$ objdump -d a.exe | grep _a\.F
 1053cb0:	e8 8b 00 00 00 	callq	139 <_a.F[.shape.*uint8]>
 1053ce9:	e8 52 00 00 00 	callq	82 <_a.F[.shape.*uint8]>
_a.F[.shape.*uint8]:
 1053d90:	e8 ab ff ff ff 	callq	-85 <_a.F[.shape.*uint8]>
 1053dc9:	e8 72 ff ff ff 	callq	-142 <_a.F[.shape.*uint8]>

Change-Id: I627f7e50210aabe4a10d0e2717d87b75ac82e99b
Reviewed-on: https://go-review.googlesource.com/c/go/+/339595


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>