split `Function` into `FunctionParameters` and `FunctionStencil`, so the
stencil can be compiled and cached, and compilation parameters can be
applied onto the result of compiling a FunctionStencil
(CompiledCodeBase<Stencil>) later on to become a consumable
CompiledCode.

…tial-eq

Unfortunately the previous implementation was more powerful, in the
sense that a function call (in a function body) could change a call
target, and that would result in a cache hit. With the new
FunctionStencil approach this is not the case anymore because that
changes the `FunctionStencil` identity. A future refactoring could make
it possible to support caching for that specific case.