Build failure: Invalid shifts (linux/amd64, Go 1.12.7)

[ajstarks]

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Using Go 1.12.7 on Fedora 30:

$ go get  github.com/MichaelTJones/gg
# github.com/MichaelTJones/lex
gowork/src/github.com/MichaelTJones/lex/lex.go:943:19: invalid operation: 1 << a (shift count type rune, must be unsigned integer)
gowork/src/github.com/MichaelTJones/lex/lex.go:945:19: invalid operation: 1 << (a - 64) (shift count type rune, must be unsigned integer)
$ go version
go version go1.12.7 linux/amd64

(incidentially I had to install bzr because of the launchpad.net/gommap package. You may want to point this out in the README)