feat(lua): add mk_metavar to Lua API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(lua): add to_nonnull_expr

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(lua): expose environment objects in the Lua bindings

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/type_checker): add missing test, and kernel_exception has_no_type_exception

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(lua): allow lua scripts (embedded in Lean files) to access the environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(emplace_back): use emplace_back when appropriate

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
style(lua): use C++-style cast

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
style(lua): use C++-style cast