Delete all deprecated features

[mlms13]

First things first:

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

Then the deletions:

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