Initial implementation of convenience pass.

[paul-tqh-nguyen]

This is intended to help with https://github.com/metagraph-dev/mlir-graphblas/issues/152.

This PR is intended to partially implement the convenience pass as described in https://github.com/metagraph-dev/mlir-graphblas/issues/152#issuecomment-963253778. This simply adds the graphblas.convert_layout adding functionality to the --graphblas-structuralize pass.

The long-term plan is to:

• merge this PR to get any misc. concerns addressed
• go through all the ops and implement the appropriate graphblas.convert_layout adding functionality (ideally with functional FileCheck tests)
• start working on the optimization pass described here

I believe the plan above will completely address https://github.com/metagraph-dev/mlir-graphblas/issues/152.

[coveralls]

Pull Request Test Coverage Report for Build 1447669674

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.8%

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Totals
Change from base Build 1446240513:	0.0%
Covered Lines:	3678
Relevant Lines:	4389

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💛 - Coveralls