Support non-interactive completions

[geraldus]

Let's:

• hardcode completion candidates for keywords
• introduce haskell-completion-at-point (language, pragma name, options, extensions, keywords) in haskell-completions.el
• add it to completion-at-point-functions in haskell-mode.el

For interactive part:

• rename haskell-completions-sync-completions-at-point to haskell-sync-repl-completion-at-point
• use haskell-completion-at-point internally
• replace haskell-completions-at-point-function in completion-at-point-functions with haskell-sync-repl-completions-at-point

I'd prefer a bit longer names: haskell-completion-completion-at-point and haskell-completions-sync-repl-completions-at-point.

Docs:

• take a look at docs, and make necessary changes and clarifications.
  • find out if section 4.2.5 is relevant ([Discussion] Deprecate haskell-mode-contextual-space #1182)
  • find out contextual-space usage ([Discussion] Deprecate haskell-mode-contextual-space #1182)
  • update section 4.3:
    • mention non-interactive completions
    • mention keyword completions
    • mention requirement of active process (session associated to buffer)

Related #215 #1169 #1184

[geraldus]

@gracjan can you take care about section 4.3 of manual?

[gracjan]

Okay. Maybe not that soon, but yes...

[geraldus]

Cool, then I can proceed with some other stuff.