make sure W_RootStructure has no attributes
introduce some abstraction for reading fields
add error-raising default implementation of arithmetic functions
Revert "introduce some abstraction for reading fields"

This reverts commit 5da15522b2450c1d673b6b5f779bf6e2b5c312ef.
introduce some abstraction for reading fields
do better things type-wise about W_Cells
no upcasting for attributes
only call set_val on W_Cells
only call extend on W_Parameterization
gah, caselambdas were rather slow

they formatted the error for the non-matching lambdas
more precise bool check
make the W_Symbol cache use the utf-8 encoded value

this saves a decoding step in the case where the symbol already exists