Convert new AST into MLIR

[rengolin]

First iteration using opaque MLIR and see what comes out of it. We still won't be able to lower that in any helpful way before we have a minimal dialect and the type inference working.

Acceptance criteria:

• Take examples that are already parsed by the front-end and convert the whole AST into MLIR
• Create some tests to show the conversion into some "expected" format
• Add necessary nodes and types to the Verona dialect that make the process simpler

Not included:

• Working on all examples before the parser can generate an AST for them
• A complete Verona dialect (Design a Verona MLIR dialect #106)

[rengolin]

Will continue working on https://github.com/rengolin/verona/tree/mlir branch, squashed from mlir-ast branch in #107.

[rengolin]

Skeleton implementation of verona-mlir in https://github.com/rengolin/verona/tree/verona-mlir.

Doesn't parse any AST, but has MLIR -> {MLIR, LLVM} structure, uses verona dialect and has some tests.

Next step, start looking at some basic AST constructs and lower to dialect IR.

[rengolin]

First prototype merged (#155) and improved (#175, #177).

[rengolin]

Now that the MLIR generator is working on basic constructs, and we have started working on the Verona dialect (#106) and its type system (#104), we should start moving from opaque lowering to Verona dialect lowering (#107). I'll close this ticket and start working on that one.