Naming convention of built-in namespaces

[dalance]

Now there are two built-in namespace: $sv and std. One is prefixed by $ and another isn't, it is inconsistent.

There are two ways to unified naming convention.

• $sv and $std: This is more clear that $ means built-in namespace, but $std is bit long and noisy for the frequent usage.
• sv and std: Opposite to the above.

At the moment, I prefer sv and std.

[dalance]

From the point of view of syntax complexity, removing $ is good because DollarIdentifier can be removed from ScopedIdentifier.

[dalance]

Refs #911 Using $ to indicate compiler built-in is reasonable. So $sv and $std is better.