Rename Type to Decoder, InputType to Encoder - #1485

I think it was good to get rid of the "input*" names here, but I'm not convinced about these "encode*" names.

I'm still not sure about the encode{Field,Constructor}With names. It's really a bit unfair that the FromDhall side got to use field and constructor!

Would constructorEncoder[With] and fieldEncoder[With] be any better? At least it would keep the encode name free for better uses…

We could also consider creating separate modules for encoding and decoding. Then we could have field[With] and constructor[With], and union and record in both modules.

To decrease the unfairness, how about names like to{Record, Field, Singleton}With for the ToDhall side and from{Record, Field, Singleton}With for FromDhall?

About module for encoding and decoding, it's an out-there suggestion but, the way tomland encodes and decodes toml is with a bi-directional codec, something like

data Codec a = 
    Codec { fromDhall  :: Expr Src Void -> Extractor Src Void a
            toDhall :: a -> Expr Src Void
            dhallType :: Expr Src Void
            }

It's pretty nice to keep the symmetry, and this way you always have to implement both encoders and decoders at the same time.

{to,from}Record don't seem too helpful to me since we have records on both sides. Similarly for {to,from}Field.

The Codec idea is intriguing, but this PR is not the right place for discussing it IMHO!

Let's maybe leave the lack of symmetry between record and union decoders and encoders for future work.