[mlir][sparse] Cleaning up names in {Merger,LoopEmitter,CodegenEnv}.{h,cpp}

This change does a bunch of renaming to clear up confusions in these files.  In particular, this change:

* Renames variables and methods to clarify the "dim"/"lvl" distinction, and changes them to use the `Dimension`/`Level` types as appropriate.
* Introduces new typedefs
  * `ExprId`, `LatPointId`, `LatSetId`: to clarify the interning design of the Merger.
  * `LoopId`, `LoopOrd`: to clarify the distinction between arbitrary names for loop-variables, vs numeric identifiers based on the actual order of loop generation.
  * `TensorId`
  * (Future CLs will change these from typedefs to structs/classes, so that the typechecker can help avoid mixups.)
* Updates documentation to match the new terminology
* Adds additional assertions
* Adds `const` to local variables along the way

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D145756