BREAKING CHANGE:
This change renames all instances of PipeRegistry to Pipes.
As part of this change, the former "defaultPipes" export is
now a Pipes instance, instead of a map. The map that was previously
called "defaultPipes" no longer exists, but may be accessed via
defaultPipes.config.
This is its own commit so that tools have an easier time of preserving
history of the file, by keeping the diff between pipes.ts and
pipe_registry.ts minimal.
Also moved pipe_registry_spec.ts
