[CoreIR] Make coreir an optional dependency

[leonardt]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.08%. Comparing base (3d1fb5d) to head (7128c5e).
Report is 1 commits behind head on master.

❗ Current head 7128c5e differs from pull request most recent head 4492cfb. Consider uploading reports for the commit 4492cfb to get more accurate results

Files	Patch %	Lines
magma/primitives/register.py	75.00%	2 Missing ⚠️
magma/util.py	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1312      +/-   ##
==========================================
- Coverage   85.09%   85.08%   -0.02%     
==========================================
  Files         169      169              
  Lines       17627    17637      +10     
==========================================
+ Hits        15000    15006       +6     
- Misses       2627     2631       +4     

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[leonardt]

@rsetaluri this is ready for review now, I updated to be a minimal change that:

• Removes the dependency from the setup.py
• Updates the internals to only import coreir on demand (so importing magma doesn't fail when coreir isn't present)

I started down the road of updating the tests, but realized it might be simpler to just do a simple sanity check for the optional dependency for now (rather than updating the tests to skip coreir if it's not present, I think it's better to migrate or remove tests we don't want anymore in a follow up PR).