[mlir][sparse] Renaming the STEA field dimLevelType to lvlTypes

This commit is part of the migration of towards the new STEA syntax/design.  In particular, this commit includes the following changes:
* Renaming compiler-internal functions/methods:
  * `SparseTensorEncodingAttr::{getDimLevelType => getLvlTypes}`
  * `Merger::{getDimLevelType => getLvlType}` (for consistency)
  * `sparse_tensor::{getDimLevelType => buildLevelType}` (to help reduce confusion vs actual getter methods)
* Renaming external facets to match:
  * the STEA parser and printer
  * the C and Python bindings
  * PyTACO

However, the actual renaming of the `DimLevelType` itself (along with all the "dlt" names) will be handled in a separate commit.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D150330

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

@@ -1159,7 +1159,7 @@ public:
     // TODO: We should check these in ExtractSliceOp::verify.
     if (!srcEnc || !dstEnc || !dstEnc.isSlice())
       return failure();
-    assert(srcEnc.getDimLevelType() == dstEnc.getDimLevelType());
+    assert(srcEnc.getLvlTypes() == dstEnc.getLvlTypes());
     assert(srcEnc.getDimOrdering() == dstEnc.getDimOrdering());
     assert(srcEnc.getHigherOrdering() == dstEnc.getHigherOrdering());
     assert(srcEnc.getPosWidth() == dstEnc.getPosWidth());