make `a=>b` syntax for `Pair(a,b)`

use Dict(iter) or Dict(::Pair...) as Dict constructors

ref #6739
combine Base.Pair and Base.Collections.Pair

add comparisons for Pair

start fixing tests
fix a mistake I made in priorityqueue in the last commit

add isless for Pair
Merge branch 'master' of github.com:JuliaLang/julia into jb/dictsyntax
Merge branch 'master' of github.com:JuliaLang/julia into jb/dictsyntax
allow parsing => as an identifier, and make it an alias for Pair

this way `a=>b` is lowered like every other operator call, except in
the old dict syntax during the deprecation period.

this is probably a good way to deal with it, and we can later decide to
remove the `Pair` alias if we want to.
update Dict show() to use the new standard syntax
deprecate old dict syntax and change uses in base and tests
update NEWS and docs for dict syntax change
Merge pull request #8521 from JuliaLang/jb/dictsyntax

RFC: regularize Dict syntax