ds::extractPairs() can not handle nested expressions

[paulhoux]

In string_util.cpp, the function ds::extractPairs is not able to handle the following expression:
#expr{ #expr{ 300 * 0.7 } * 0.7 } with leftDelim = #expr{ and rightDelim = }.

The above expression can occur in the following case:

$_title_font_size = 300
$_header_font_size = #expr{ $_title_font_size * 0.7 }
$_body_font_size = #expr{ $header_font_size * 0.7 } # <-- there it is

It incorrectly stops at the first right bracket } and returns #expr{ 300 * 0.7 as a subvalue. Nested expressions should be detected and in this case #expr{ 300 * 0.7 } * 0.7 should be returned.

When fixing this issue, be careful not to break support for the following unnested example: #expr{ 200 + 50 }, #expr{ 300 + 100 }

[paulhoux]

While you're at it, consider using output.emplace_back(0, thing) instead of output.push_back(std::make_pair(0, thing))