debug/goobj: add String methods for SymID and SymKind
debug/goobj: add String methods for SymID and SymKind

R=iant
CC=golang-codereviews
https://codereview.appspot.com/48890044

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme