OutlineCfg use insert_hugr; remove CfgBuilder::from_existing

[acl-cqc]

This requires copying the created nodes, but there are, like, 6 of them, and means we can drop CfgBuilder::from_existing, easing the path to #282.

Also merge identical-signature impl Blocks and add BlockBuilder::finish_hugr_with_outputs, but no other Builder refactoring was required :)

[acl-cqc]

There is another option/possibility that I thought about: adding to HugrMut new methods block_builder, dfg_builder, cfg_builder etc. (taking a parent node, and whatever inputs/Signature/etc. as per the corresponding methods in DfgBuilder/CfgBuilder) that return a BlockBuilder/DfgBuilder/CfgBuilder/whatever<&mut Hugr>.

At present the implementations of these builder methods in Dataflow depend upon Container so would need reworking, but since the impl of HugrMut knows it has an &mut Hugr it could be done - at least until we impl HugrMut for FlatRegionView whereupon we need the builder classes to work on a FlatRegionView, so I didn't think this would work out in the longer term.

[ss2165]

👍