Deprecate Dict.update/3 in favor of Dict.update!/3

A non-raising version that yields nil will be added in
upcoming releases.
Macro.unescape_binary -> Macro.unescape_string
Move from unicode to binary to String.from_char_list
Move from unicode to list to String.to_char_list
Update CHANGELOG
Do not compile behaviour docs if --docs is not set
Write test cases for doctest indentation

These are the examples reported in issue #1350.
Write code for doctest indentation
Add "preceding text" test cases for doctest indentation
Rewrite code for doctest indentation

This combines the check_indent and measure_indent into a single pass.
Remove trailing whitespace in Dict