Define constants for sizes of Sym structures.
Define constants for sizes of Sym structures.

R=rsc, agl
http://go/go-review/1016028

pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,tv42/old-go,dsriram/golang,bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,pombredanne/go-deleteme,nightlyone/golang,nightlyone/golang,h8liu/golang,bdwalton/debian-golang,tv42/old-go,ssrl/go,bdwalton/debian-golang,dsriram/golang,gnanderson/go,nightlyone/golang,pombredanne/go-deleteme,gnanderson/go,h8liu/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tianon/debian-golang,tianon/debian-golang,bdwalton/debian-golang,tianon/debian-golang,nightlyone/golang,h8liu/golang,gnanderson/go,gnanderson/go,dsriram/golang,h8liu/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,h8liu/golang,dsriram/golang,tv42/old-go,tv42/old-go,ssrl/go,bdwalton/debian-golang,pombredanne/go-deleteme,nightlyone/golang,tv42/old-go,dsriram/golang,tv42/old-go,gnanderson/go,tianon/debian-golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,ssrl/go,nightlyone/golang,ssrl/go,dsriram/golang,tianon/debian-golang,h8liu/golang,bdwalton/debian-golang,apparentlymart/golang,ssrl/go,bdwalton/debian-golang,h8liu/golang,tv42/old-go,bdwalton/debian-golang,nightlyone/golang,ssrl/go,tianon/debian-golang,apparentlymart/golang,tianon/debian-golang