[concept] Add concept check utilities

The main feature of this header is the constexpr function
models(). The result of

  models<C, T1, ..., TN>()

indicated if the concept C is modeled by the collection
(T1, ..., TN) of types. A concept is a list of valid
expressions that a type must support. Concepts can refine
other concepts to inherit all requirements from those
base concepts.

Besides the concept check function itself, the header contains
some helper functions of the form require requireFoo for additional
checks. The purpose of those functions is to transform a non-satisfied
condition Foo to a substitution failure, such that the corresponding
expression is not valid. E.g.,

  requireConvertible<R>(t.f())

is not valid unless the result of t.f() is convertible to R.