Allow global reads

[tam7t]

$ pancakeinfo -pkg unicode -pure=false
IsNumber              basic /usr/local/go/src/unicode/graphic.go:108:26
ToUpper               calls unicode.To
IsSpace               basic /usr/local/go/src/unicode/graphic.go:135:26
...
IsLetter              basic /usr/local/go/src/unicode/graphic.go:94:26

Basically all of these are pure so long as the global variables are never written to. Instead of special casing all of these with DefaultIgnoreRead, analyze should only mark global reads as impure when there is a path to a global write.