fix(library/standard): make sure file can be compiled when processing theorems in parallel

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

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): copy rec and initial fields when processing Exprs and ScopedExpr

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add hilbert's choice operator

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add hilbert's choice operator