refactor(library/data/nat/basic): remove unnecessary nat_

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add 'as' clause to 'open' command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add 'export' command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/declaration_index): store declarations in a map

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
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