using unaryFun in functions that take a comparator (eg sort etc): sort!"a[0]" <=> sort!"a[0]<b[0]"

[Timothee Cour]

very often I would wish that sort (+ related functions) could take unaryFun
arguments and convert them to binaryFun as follows:

//pseudocode:
template unaryToBinaryComp(alias foo) {
bool unaryToBinaryComp(T)(T a, T b) if (__traits(compiles,foo(a) < foo(b))
) {
return foo(a) < foo(b);
}
}


Using this we could support much nicer syntax for sorting with unary
functions, for example:

sort!"foo(a)" <=>sort!(unaryToBinaryComp!(unaryFun!"foo(a)"))
<=> sort!"foo(a) < foo(a)"
sorting in reverse order is easy: just use sort!"-foo(a)" (works for
signed, unsigned, floating point types etc).

Examples of use:

E1)

struct A{
double score;
int index;
}

[A.init,A.init].sort!"a.score";
instead of:
[A.init,A.init].sort!"a.score < b.score";

E2)

mytuple.sort!"a[0]"
instead of:
[A.init,A.init].sort!"a[0]<b[0]"

I find that in large majority of cases, binary fun is used in that way. So
let's support both unaryFun and binaryFun.
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