xdrgen: Keep track of on-the-wire data type widths

The generic parts of the RPC layer need to know the widths (in
XDR_UNIT increments) of the XDR data types defined for each
protocol.

As a first step, add dictionaries to keep track of the symbolic and
actual maximum XDR width of XDR types.

This makes it straightforward to look up the width of a type by its
name. The built-in dictionaries are pre-loaded with the widths of
the built-in XDR types as defined in RFC 4506.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>