Add a DEBUG mode

[vkhodygo]

Current code contains too many checks, including the case of nested functions, going through the same data again and again.
Some (all?) of them significantly affect the performance and should be optional.
This could be done via:

• an environment variable, the default state is to check everything;
• an extra boolean parameter passed to every function. This is a more flexible option, however, this leads to combinatorial explosion of the total number of tests.