Coder + multiline string improvements. Bugfixes

Breaking Changes

* as_dhall on array now flattens nested unions if they would be generated
* as_dhall now uses Dhallish names for tags on generated unions for all
elements that can be represented untagged
* as_dhall only adds hash to tag name when needed
* Coder treats all Dhallish tags as extractable instead of trying to revive
* Coder strips any underscore-then-sha256hex suffix from a tag when reviving

Bugfixes

* Coder decode on Proc works now
* Fix for substitution on LetBlock/LetIn
* Fix typechecking partially-applied builtins
* Fix "stack level too deep" on long multiline comment
* Fix for typechecking lists where elements had different, but compatible,
types
* Fix for IP address parsing in imports (from upstream)

New Features

* Tabs and blank lines allowed in multiline strings (standard version
v7.0.0-21-g092996d now supported)
* Self-describe CBOR tag allowed anywhere
* as_dhall for Proc, cannot be serialized but useful for calling Dhall function
* dhall-compile: new command to resolve, normalize, and seralize whole
directories of Dhall code