introduce a Pair API (alternative to make)

[rfourquet]

This make the following API available:

• rand(T => x) is equivalent to rand(make(T, x))
• rand(T => (x, y, ...)) is equivalent to rand(make(T, x, y, ...))

[mschauer]

Wouldn't the order x=>T be more natural, as in x::T?

[rfourquet]

Ah interesting, didn't think about it. The T => x order reads for me as "generate a T, and here is/are the parameters to do it. It also match the order in make, and puts the emphasize on what type has to be built. But x => T reads as "from this/these values (arguments), construct a T.

[rfourquet]

I will rebase and merge. I still prefer the T => x syntax. This can still be changed later if there is support. This will be considered experimental at this point, and library code should only use make (while the => syntax can be used in the REPL or throwaway scripts).

[rfourquet]

Another small argument in favor of the T => x order is the right-associativity of =>. Indeed, x itself can be a Pair, but not T. So if x is a Pair S => y, then T => S => y is equal to T => (S => y) == T => x. E.g.:

julia> rand(NTuple{4} => Complex => 1:3)
(2 + 2im, 2 + 3im, 1 + 3im, 2 + 1im)

(this example does not work yet with this PR, patch coming).