Fix another bug in which scope of data is passed to data="all"
soyhtml: don't write "__all" variable to user's maps.
ast/MsgNode: add "meaning" attribute
{msg}: do not allow msg or control structures within msg tags
ast/parse: add MsgPlaceholderNode
soymsg, soyhtml: support for raw text messages

- soymgs.Message and .Bundle interfaces
- bundle may be provided to soyhtml.Renderer
- {msg}s will be looked up in the bundle
- message id calculation ported from official soy
XXX starting on msg datarefs