fix(library/unifier): bug in unifier priority queue

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/elaborator): case-split on coercions that cannot be resolved by postponing

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): try overloaded notation and declarations in the order they were defined

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): change the name resolution rules: when in a namespace N that defines C, then C always refers to N.C (i.e., it overrides any alias)

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