fixes#5395
Previously values of `const` statements used the same scope as the
`const` statement itself, meaning variables could be declared inside
them and referred to in other statements in the same block. Now each
`const` value opens its own scope, so any variable declared in the value
of a constant can only be accessed for that constant.
We could change this to open a new scope for the `const` *section*
rather than each constant, so the variables can be used in other
constants, but I'm not sure if this is sound.
