layout: Make line breaking able to restart from any position.

This commit removes the "merge-fragments" pass from inline reflow,
instead merging "on the fly". This ended up being simpler, as well as
more fine grained. Additionally, this patch makes the line breaker no
longer clone every fragment (!)

This functionality will be used in the implementation of
`text-overflow`.
auto merge of #4371 : pcwalton/servo/line-breaking-improvements, r=mbrubeck

This commit removes the "merge-fragments" pass from inline reflow,
instead merging "on the fly". This ended up being simpler, as well as
more fine grained. Additionally, this patch makes the line breaker no
longer clone every fragment (!)

This functionality will be used in the implementation of
`text-overflow`.

r? @mbrubeck