add typename mangling

Three options for what to do with long type names:

M_NONE - original, default. Do nothing special.

M_STRIP_PACKAGE - Strip local package name. Useful when you want to use
package names, but don't want to deal with
`com_mycompany_supersoftware_proto_` prefixes on every symbol.
Not useful when your packages cross-reference each other, because it only
strips the _current_ package name, so references to other packages
actually become invalid.

M_FLATTEN - Always use only the last part of a dotted name.
Useful when you have packages and/or deeply nested structs that don't
actually have colliding names. Then you get shorter C symbols for no
additional cost.
Not useful when the names can collide.