Cleanup qupulse.program's `__init__`

[shumpohl]

The __init__.py of qupulse.program contains several class definitions and the number grew over time. This easily leads to circular imports when new features are added. Therefore, we will

• rename SimpleExpression to DynamicValue and move it to the new module qupulse.program.values. The rename is to make clear that it is supposed to act as a value-like object and not an expression like object.
• Move ProgramBuilder and Program protocols to qupulse.program.protocol following the example set by the qupulse.expression sub-package

[shumpohl]

Done in #908