Introduce PsLiteral and PsLiteralExpr.

docs/source/backend/index.rst

docs/source/backend/objects.rst

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/emission.py

src/pystencils/backend/functions.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/literals.py

src/pystencils/backend/platforms/generic_gpu.py

...pystencils/backend/transformations/eliminate_constants.py

...ils/backend/transformations/hoist_loop_invariant_decls.py