collapse consecutive if-statements with the same condition (#450)

[Rich-Harris]

Ref #450. Adds a new addConditionalLine method to CodeBuilder which collapses things like this...

if ( foo ) {
  bar();
}

if ( foo ) {
  baz();
}

...into things like this:

if ( foo ) {
  bar();
  baz();
}

Only used inside recompute for now, but could probably be used elsewhere as well.