Scopes: make sure every bucket of the hash table is used

[smarter]

Previously, one bucket was never used because in binary,
12*2^n - 1 = 101111...

Review by @odersky @namin .

[smarter]

By the way, Scopes.scala seems to have some vestigial comments and code, the description of MutableScope says:

Note: constructor is protected to force everyone to use the factory methods newScope or newNestedScope instead.
This is necessary because when run from reflection every scope needs to have a
SynchronizedScope as mixin.

But SynchronizedScope does not exist in Dotty.
Similarly, the method newNestedScope is never used.

[odersky]

LGTM