Introduce support for Infinities and Transcendentals. Document supported SymPy features.

This MR introduces parsing and code generation for pi, e, and positive/negative infinity. Furthermore, the documentation on constants and functions in the backend, as well as supported SymPy features is extended.

Infinities and Transcendentals

• Introduce PsConstantFunction subclass of PsFunction to represent typed special constants (pi, e, infinities)
• Add freeze support for sp.pi, sp.e and sp.oo
• Extend typifier; treat PsConstantFunction like PsConstants in contextual typing
• Extend vectorizer; broadcast PsConstantFunctions
• Add support for emitting special constants to CPU/x86 and CUDA/HIP targets

Documentation

• Explain design and interactions of PsConstant, PsFunction and PsLiteral in the backend guide
• Add a list of supported SymPy features to the API docs