internal/godoc: register all go/ast nodes with gob
internal/godoc: register all go/ast nodes with gob

Change-Id: I68c2874345525407fbf414f5cd7e13966c2a36a3
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259097
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite