add "e" and "pi" aliases to libc.math declarations to bring it closer to a drop-in replacement for Python's math module

silence a C compiler warning for trivial generators that do not use their closure class cast (seems to be difficult to avoid such cases)

make new feature-specific guards overridable by user #defines in CPython to allow switching them on and off for testing/debugging/benchmarking

replace generic "COMPILING_IN_*" C macros with feature specific guards that allow a more fine-grained adaptation to C-API implementations

speed up delegation of generators and coroutines a little by avoiding an indirect C call in favour of a directly visible recursive call

make current scope available to FunctionState and CCodeWriter to give access to variable types from AnnotationCCodeWriter

Fix syntax error in memoryview userguide documentation.