contracts: Fix double charge of gas for host functions (#3361)

This PR is fixing a bug in the sync mechanism between wasmi and
pallet-contracts. This bug leads to essentially double charging all the
gas that was used during the execution of the host function. When the
`call` host function is used for recursion this will lead to a quadratic
amount of gas consumption with regard to the nesting depth.We also took
the chance to refactor the code in question and improve the rust docs.

The bug was caused by not updating `GasMeter::executor_consumed`
(previously `engine_consumed`) when leaving the host function. This lead
to the value being stale (too low) when entering another host function.

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Co-authored-by: PG Herveou <[email protected]>