Fix #1186: unset line-height for code spans
Fix #1186: unset line-height for code spans

As described in the issue, previously, the spec would scroll up while
syntax highlighting was being applied (a lengthy process especially in
the html-single version). This happened because as the syntax
highlighting is applied, parts of the code (a <pre> block) are wrapped in
<span>s, which (unlike <pre>s) receive a line-height of 1.40em (the
default is unspecified, but according to MDN roughly 1.2); highlighted
code is therefore spaced apart further and takes up more screen space.

Instead of removing the line-height declaration altogether (as I proposed
as alternative 3 in the issue), which also affects tables (arguably
negatively), we simply override it only in <span>s within <pre>s, within
a rule that already contained another code-specific declaration.

jvasileff/ceylon-spec,jvasileff/ceylon-spec,ceylon/ceylon-spec,jvasileff/ceylon-spec,ceylon/ceylon-spec,ceylon/ceylon-spec