[dev.typeparams] cmd/compile: split package escape into multiple files

[dev.typeparams] cmd/compile: split package escape into multiple files

This CL reorganizes the code from package escape into multiple files,
so the relationships between bits of code are hopefully easier to
follow. Besides moving code around and adding necessary
copyright/import declarations, no code is touched at all.

Change-Id: Iddd396c3a140f4eb1a7a6266d92a4098118b575b
Reviewed-on: https://go-review.googlesource.com/c/go/+/329989
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>