Merge branch 'mk/system_api_improvements3' into 'master'

chore: RUN-521: Additional system api simplifications

With this MR we end up with three functions for charging for api calls:

charge_for_cpu - which is used in cases where we only have cpu cost

charge_for_cpu_and_mem - which is used in cases where we touch memory

charge_for_stable_write - which name is self explanatory

Additionally, minor improvements and refactors. Among others, some shuffling around allowed to reduce the number of times system_api is extracted from store (which unwraps the option) 

See merge request dfinity-lab/public/ic!15627