Handle block of blank lines for code folding properly (fixes #1707).

[torimus]

Consider context of consequent blank lines, to which code & indentation level it belongs.
Leaves blank lines of outer indentation levels intact.

Fixes issue #1707.

[purcell]

Thanks for this! Looks reasonable - I noted a couple of minor issues.

I wonder what you think of my suggestion in the linked issue that we retain a single blank line?

[purcell]

A quick fix to follow docstring conventions:

Suggested change

	"returns `t' if following sequence of blank lines fits specific indentation level"
	"Return non-nil if sequence of blank lines in direction DIR fits given INDENT level."

[purcell]

Please use spaces for indents.

[purcell]

This progn wrapper is redundant in the else clause of if statements, and it would be more idiomatic to omit it.

[torimus]

@purcell

A quick fix to follow docstring conventions

Thanks for the correction, fixed in a new commit.

Please use spaces for indents

Oh sorry, wasn't aware emacs doesn't automatically sets spaces-only for elisp mode indentation like used in other language modes. Fixed.

This progn wrapper is redundant in the else clause of if statements, and it would be more idiomatic to omit it.

Right. I'm not learned in Emacs Lisp, now found out in docs else branch is not limited to a single expression.

I wonder what you think of my suggestion in the linked issue that we retain a single blank line?

Not sure if I follow you correctly, but this PR introduces 3 new failed tests in haskell-collapse-tests.el
FAILED test-haskell-indented-block-1
FAILED test-haskell-indented-block-3
FAILED test-haskell-indented-block-7

which all depend how would you expect correct code folding. In my opinion, retained blank line(s) between code blocks at the same indentation level(as the initial) are to be expected. Only collapse them in nested indentation levels. I can "fix" tests appropriately to correspond to aforementioned logic so it will pass.