Make amount constants untyped.

Since these constants can be useful for int64, Amount, and float64
math, it doesn't make sense to make them just one type, and require
type conversions for the rest.

ok @davecgh
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.