Indent "let", "when" and "while" as function form if not at start

[Macroz]

"let", "when" and "while" are considered to be a macro form normally:

(when foo
  bar)

Also you can introduce macros that start with "let", "when" or "while" that
will be indented like macro forms:

(when-foo-bar
  1
  2
  3)

But when "let", "when" and "while" are not in the beginning of a symbol they are
now threated as function forms:

(foo-when-bar 1
              2
              3)

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Thanks!

[bbatsov]

I guess it's also worth adding some explanations in the code here, as to what it's aiming to achieve. You should also check if this won't break if-let.

[Macroz]

Are the new explanations something you had in mind?

I also added tests for if-let case and fixed it.

[bbatsov]

Thanks!