Support ES2015 block-level function declarations in strict-mode

[p-bakker]

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/block#block_scoping_rules_with_let_const_or_function_declaration_in_strict_mode

In ES5 function declarations were not allowed in strict mode in the newly introduced block scopes
As of ES2015 function declarations inside block scopes in strict mode are allowed, but they are to be block scopes

Note that our let implementation already adheres to block scoping (while our const impl. does not, see #939)