Fix alignment of Source links in instance table in Firefox

Due to a Firefox bug [1], a combination of 'whitespace: nowrap' on the
parent element with 'float: right' on the inner element can cause the
floated element to be displaced downwards for no apparent reason.

To work around this, the left side is wrapped in its own <span> and set
to 'float: left'.  As a precautionary measure to prevent the parent
element from collapsing entirely, we also add the classic "clearfix"
hack.  The latter is not strictly needed but it helps prevent bugs if
the layout is altered again in the future.

Fixes #384.

Remark: line 159 of src/Haddock/Backends/Xhtml/Layout.hs was indented to
        prevent confusion over the operator precedence of (<+>) vs (<<).

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=488725