Enhancement: Add greek letter version of constants

[orelbn]

Currently, pi can be evaluated as constant:

But the Greek version can not be:

It's the same situation for phi.

It would be nice to be able to use the Greek letter equivalent as constants as well, without needing to set it explicitly.

[josdejong]

The library restricts itself to ascii characters, though it allows using non-ascii characters like π. But indeed, you can load aliases yourself into the library so there is no limitation in that regard.