perf(library/type_inferer): improve is_proposition performance

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/elaborator): avoid exception

Lean was spending 17% on the runtime "throwing exceptions" in the test tests/lean/implicit7.lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): bug in process_metavar_inst, and disable simplification that is negatively impacting the elaborator

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/elaborator): avoid exception

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