tests/bench: Time namedtuple field access.

That's higher than instance field access - behold the power of hashing.
tests/bench: Add testcases for lookup in 5-el instance and namedtuple.

... and we have not that bad mapping type after all - lookup time is ~ the
same as in one-attr instance. My namedtuple implementation on the other
hand degrades awfully.

So, need to rework it. First observation is that named tuple fields are
accessed as attributes, so all names are interned at the program start.
Then, really should store field array as qstr[], and do quick 32/64 bit
scan thru it.
tests/bench: Add variation on loop_count/while_down_ne test.
tests/bench: Add tests for various ways to pass function args.

Passing 3 args with keywords is for example 50% slower than via positional
args.
tests/bench: Add testcase for positional/kwargs to enumerate().

Inspired by discussion in #577. So, in this case of builtin function,
passing args by keyword has less than 1% overhead.