closes #633

* Add test case. Reduced this to a 2-node DFG (plus Input and Output).
That this hasn't been showing up repeatedly in other tests - I put down
to #388: most custom ops have empty `extension_reqs` so do not generate
Plus constraints.
* Run (50%-failing) test 10 times -> failure rate ~~ 1 in 2^10
* Fix calculation of `live_vars` and `live_metas` by one-off traversal
of constraints+solutions in `fn results()`.

---------

Co-authored-by: Craig Roy <craig.roy@cambridgequantum.com>

* Make each Value able to report its ExtensionSet needed at runtime (and
hence, also each CustomConst)
* Also give OpTrait a method for the extension-delta. This is the delta
of the FunctionType, *for dataflow ops*, but can be defined for other
optypes too - specifically (here), Const ops and also Case. Use this in
both inference and for `NodeType::io_extensions()`
* Input extensions of every Const node can now be the empty set (i.e.
the default-for-ModuleOp `pure`), as the relevant extension will be
added in the delta - thus, drop separate extension parameter in builder
(add_constant, add_load_const, etc.). LoadConstant ops can now also have
an empty delta and open input-extensions as these can be figured out
from the Const. This is thus a step towards fixing #702...
* Also note slight issue in replace::test::cfg, pending #388
* Add `ExtensionSet::union_over(impl IntoIterator<Item=Self>) -> Self`
utility method

This should ease the way towards solving the rest of #702 and moreover
to removing `new_auto` - we should be able to constrain the
input-extensions for any ModuleOp to `pure` in inference and thus
*every* node can be created open rather than a mix, but those are all
for follow-up PRs.

…RIP inference (#1142)

This is probably phase 1 of 4, later steps being
* Remove `input_extensions` and `NodeType`
* Make every operation require it's own extension (#388)
* Infer deltas for parent nodes (#640) - this may be disruptive, and/or
take some subtlety, as currently every FunctionType stores an
ExtensionSet not an Option thereof
* Finally, remove the feature flag

So, this PR updates validation to ignore the input_extensions, and
removes the inference algorithm that would set them. There were a few
complications:
* A lot of tests create a DFGBuilder with an *empty* delta, then put
things in it that have extension requirements. The inference algorithm
figures out that this is all OK if it can just put that
extension-requirement on the `input_extensions` to the DFG node (root)
itself. So this might be a call for `input_extensions` - they do reduce
the need for #640 a bit.
* We can see roughly how painful life is (without delta-inference nor
input-extensions) looking at the various extra extension-deltas I've had
to specify here
* I think that given we are under the feature flag at the moment, we are
OK to continue, however the need for #640 is now somewhat increased, so
discussion there strongly encouraged, please! :)
* `TailLoop` turned out not to have an extension-delta, where it clearly
needs one (just as DataflowBlock and others). Also there was an
implementation of `OpParent` for it, whereas in fact we needed
`DataflowParent` as that would also have got us the `ValidateOp` for
free. This all in 08bb5a5.
* Another bug in DataflowBlock::inner_signature, and some others.

....That is to say, in some ways this validation scheme is *stricter*
than what we had; there is both the slight flexibility that
`input_extensions` gave us in mis-specifying deltas, and that this
scheme's simplicity makes it much harder to accidentally omit cases from
validation....

I've also kept `Hugr::infer_extensions` around as a no-op rather than
remove it and later bring it back (in #640). Maybe we should remove it,
but that would be a breaking change....OTOH I've removed
`validate_with_extension_closure` and in theory we could have that (a
closure containing deltas for nested DFGs) too...

BREAKING CHANGE: TailLoop node and associated builder functions now
require specifying an ExtensionSet; extension/validate.rs deleted; some
changes to Hugrs validated/rejected when the `extension_inference`
feature flag is turned on

Includes adding new variants of `block_builder`, `entry_builder`,
`simple_entry_builder` and `conditional_builder`: the default version
omits the extension set parameter, the `_exts` variant takes an extra
parameter (being an ExtensionSet). `simple_block_builder` is untouched
(as it takes a FunctionType, so can use `endo_ft`/`inout_ft`)

We'll need similar updates to `cfg_builder`, `tail_loop_builder`,
`ConditionalBuilder::new` and `TailLoopBuilder::new` but I'll leave
those for another PR, there's quite enough here ;)

closes #388

BREAKING CHANGE: (1) container-node extension-deltas will need to be
enlarged to include ops therein; for FuncDefn this will have to be
manually specified but for other containers TO_BE_INFERRED, `endo_ft` or
`inout_ft` all work. (2) `block_builder`, `entry_builder`,
`simple_entry_builder` and `conditional_builder` no longer take an
ExtensionSet; either drop the argument or use the `..._exts` variant.