fix: uncomplicate type definitions

[marshift]

the type definitions used in spitroast have been a touchy subject for a long time. the adjustments made by this pull request should hopefully end that. they are considerably less complex, but still robust and well-tested - i've been using them in my fork, strawberry, for a while now.

this DOES induce a file size regression, but fixes an actual runtime regression:

${String(funcName)} is not a function in ${funcParent.constructor.name}

i am now explicitly casting funcName to String, which is needed as it can be symbol which errors if implicitly cast at runtime.