refactor(frontends/lean): constant/axiom are top-level commands, parameter/variable/hypothesis/conjecture are section/context-level commands

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel): rename var_decl to constant_assumption

Motivation: it matches the notation used to declare it.
feat(frontends/lean): enforce rule section parameters cannot depend on section variables
feat(frontends/lean): section variables occur after section parameters
feat(frontends/lean): enforce new semantics for section 'variables'

The library file logic/core/connectives uses the new feature.
feat(frontends/lean): enforce new semantics for section 'variables'