Export CommandSize constant.

This commit exports the CommandSize constant to provide callers with the
ability to know the size of the command field in a bitcoin message header.
Use a more specific license adjective.
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.