[#251] Use haskeline in morley-debugger

Description

Problem: in morley-debugger we use hand-written REPL with our implementation of usual REPL features, like ^D handling or prompt. However, there exist nice libraries for REPLs already.

Solution: use haskeline library, that is the usual choice to make REPLs in Haskell. With it we gain for free, for example, history navigation and editing of the command line.

Related issue(s)

Resolves #251 (closed).

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