Only build FSharp PCL if reference assemblies found
Merge pull request #276 from rneatherway/issue275

Only build FSharp PCL if reference assemblies found
Unicode letters & operators

examples:
let ★ x = x + 5
let (|★) x y = x + y
let summ = ★ 5 |★ 5
comment about \So infix operators
apply & merge optimizations from : https://github.com/jack-pappas/fs-core-optimized , thanks to Jack Pappas
revert set.fs optimizations due complicated conflicts