in general introduced it with vector in mind, to do the masking through
multiplication... though intrinsic vector has an element-wise ternary...
so will need a special mask function here anyway, no need for the
variable.

car is a variable now, not a function,
separated tuple stuff,
tuple_car forwards get<0> better (why even?),
added tuple_tie_cdr (the real reason any of this happened).

still dubious

along with way, wayback, halfway, halfwayback and "improved" average.

with tests stolen from libc++.

tightened up the SFINAE,
removed Operator template parameter "alias" because of a weird bug in gcc 7/8,
evil implicit conversion:
	enabled for second parameter of in place ops by disabling the type deduction,
	enabled for binary operators through special "explicit" mechanism,

added remove_cvref.

for an inappropriate use case of including a file without adding the
library root to the include path.

I'm going to get bitten for not writing unit tests for it, aren't I?

an equivalent of indexing and array,
now that i'm committing it i realize that "visit" might have been a
better name

nodiscard and constepxrness tests for midpoint and average.