fix(library/tactic/goal): bug creating main proof builder

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin): add num type (the base type that will be used to build nat, int, real)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin): add num type (the base type that will be used to build nat, int, real)