perf(library/unifier): simple optimization

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/unifier): improve m_mvar_occs management

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/metavar): remove unnecessary functionality

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/unifier): use d_instantiate_metavars

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(kernel/converter): do not cache easy cases

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/metavar): add option to instantiate only expr metavars

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add basic heq theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add basic heq theorems