Add class name to JarMappingLoadTransformer field/method

Not used by CSVMappingTransformer, since the CSVs only contain:
    func_###,newname
    field_###,newname
with no class name, but it could be used by other load transformers.
Add method descriptor to JarMappingLoadTransformer

Again, not used by CSVMappingTransformer, but could be used by other
transformation classes to be written.
Add loading of numeric "srg" names from MCP directory

Normally, loadMappingsDir() loads:

    obf -> num -> pkgmcp

but with numericSrgNames, it loads:

    pkgmcp -> num

where:

obf = obfuscated "notch" names
num = numeric "srg" names
pkgmcp = descriptive "csv" names

This implementation could probably be improved further.

Compatibility Note: --numeric used to load these mappings:

    obf -> num

that is, ignoring the fields/methods.csv. But this turned out not to
be too useful. You can still get this mapping with ignoreCsv=true, but
now numericSrgNames and --numeric loads the much more useful mapping:

    pkgmcp -> num

Useful for targetting FML runtime deobfuscation.