fix!: Allow Const and FuncDecl as children of Modules, Dataflow Parents, and CFG nodes

[doug-q]

We take the opportunity to fix a bug where the promise to ignore duplicate calls to emit_func was not upheld, and to clean up the now unnecessary Emission.

Fixes #4 , although we go further than required by this issue.

BREAKING CHANGE: rename emit_global to emit_func. Replace Emission type with FatNode<FuncDefn>.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.

Project coverage is 83.12%. Comparing base (8dcba0f) to head (cbf3b74).

Files	Patch %	Lines
src/emit.rs	63.63%	2 Missing and 2 partials :warning:
src/emit/ops/cfg.rs	82.35%	1 Missing and 2 partials :warning:
src/emit/ops.rs	66.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #46 +/- ## ========================================== + Coverage 82.19% 83.12% +0.93% ========================================== Files 16 16 Lines 2190 2234 +44 Branches 2190 2234 +44 ========================================== + Hits 1800 1857 +57 + Misses 258 246 -12 + Partials 132 131 -1 ``` | [Flag](https://app.codecov.io/gh/CQCL/hugr-llvm/pull/46/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL) | Coverage Δ | | |---|---|---| | [rust](https://app.codecov.io/gh/CQCL/hugr-llvm/pull/46/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL) | `83.12% <90.52%> (+0.93%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL#carryforward-flags-in-the-pull-request-comment) to find out more.

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