Switch to classy field lenses

[fizruk]

Closes #39.

This change affects many names and types.
The goal is to make code look nicer.

Here's one example:

--- mempty
---   & schemaTitle   ?~ "Email"
---   & schemaType    .~ SwaggerString
---   & schemaFormat  ?~ "email"
+-- (mempty :: Schema)
+--   & title  ?~ "Email"
+--   & type_  .~ SwaggerString
+--   & format ?~ "email"

These names had to gain _ suffix:

• type_, in_, default_ (keywords);
• minimum_, maximum_ (conflict with Prelude);
• enum_ (conflict with Control.Lens).

[fizruk]

The downside of this change would be more ambiguous types, since names are overloaded now.
However, I can't tell how much a problem that would be.

I still have to check/update the documentation and, probably, write more about lenses and shortcut Has*/Ixed/At instances.

@dmjio, @jkarni I would appreciate you opinion/review on this PR. Note that these changes would affect servant-swagger users as well.

[jkarni]

This LGTM. It seems a lot nicer to use, and at least for my purposes ambiguity isn't an issue.

One thing to note is that jsonPrefix is a little unsafe - there's no check that the letters dropped are actually the start of the field name. This PR doesn't seem to run afoul of that, but at some point maybe using TH for the From/ToJSON instances so you can safely error out of it doesn't match might be an option.

[fizruk]

@jkarni I have deliberately switched all TH instances to Generic-based in #25.
It had to do with on of the aeson-0.10.0.0 bugs.

Instead of switching back to TH we might improve on the jsonPrefix to automatically detect to drop common prefix.

[dmjio]

👍

[fizruk]

@jkarni @dmjio this is a pretty heavy API change. Should it be released as 2.0 or 1.3?

[dmjio]

2.0, it's a big change