Delete all deprecated features

[mlms13]

First things first:

• [x] Move all tests out of the "option" and "string nel" -specific test files
• [x] Add an "Upgrading from v1" guide to the docs website
• [x] #127

Then the deletions:

• [x] Delete Decode_AsResult_OfStringNel
• [x] Delete Decode_AsOption
• [x] Delete Decode_NonEmptyList
• [x] Delete Decode_Base.Pipeline
• [ ] Delete Decode.Make
• [ ] Delete Decode_ParseError.ResultOf (and related helper modules)
• [x] Delete mapN helpers
• [x] Delete tupleN, tupleAtLeastN decoders
• [x] Delete stringMap
• [x] Delete variantFrom*

For each deletion, make sure the upgrade path is clearly outlined and that any recommended new functions are documented in a guide on the website.