Replace nRows, nCols, nMaj, nMin with dim and dimF
Clean up de/compress using dim/F
Move de/compress into FormatR

Using types to forbid compressing a Matrix C or decompressing a Matrix U
is overly restrictive because those functions should be no-ops.
Fix warnings

A number of shadowed names were underscored. It would be smart to think
carefully about public API's name scheme.