…ails for namespaced functions

add `ExprSyntaxError`

Due to the quite messy heritage of this codebase -- including a large part
of it being just a fork of my earlier personal project ZCL -- there were
many different conventions for how to pretty-print and diff values in the
tests in different parts of the codebase.

To reduce the dependency sprawl, this commit now standardizes on:
  - github.com/davecgh/go-spew for pretty-printing
  - github.com/google/go-cmp for diffing

These two dependencies were already present anyway, are the most general
out of all of the candidates, and are also already in use by at least some
of HCL's most significant callers, such as HashiCorp Terraform.

The version of go-cmp we were previously using seems to have a bug that
causes the tests to crash when run under the Go race detector, so I've
also upgraded that dependency to latest here to clear that bug.