This commit changes the delimiter between namespaces and their members
from "." to "::" in internal names. The new delimiter certainly feels
very un-JavaScript-y, but the meaning is still recognizable, and these
names aren't exposed to users in most cases anyway. Still, this is a
breaking change, as it affects the import names for `declare namespace`
members (as seen in tests/compiler/declare.js), and transform authors
will also need to update their code accordingly.

Fixes AssemblyScript#2793.

This isn't fully TS compatible, but refactors targeting internal names,
scoping, merging, etc. are needed to become more compatible. For
instance, if namespace members had unique separators in internal names,
then a non-exported namespace member would override a static class
member, assuming the names are the same.

Note that this change doesn't prevent the compiler from attempting to
compile the duplicate global, and hence the previous commit is needed
for this to work fully.

Barely fixes AssemblyScript#2793.