Deprecate dict comprehension construction in favor for Generator

[dhoegh]

As the new Generator syntax matches nicely with the construction of dictionaries. I propose using generators instead of using the comprehensions syntax to construct dictionaries as the generator syntax is closer to the syntax for manually initialization of the dictionaries. This should be deprecated:

julia> [i=>i*2 for i=1:2]
Dict{Int64,Int64} with 2 entries:
  2 => 4
  1 => 2

In favor for:

julia> Dict(i=>i*2 for i=1:2)
Dict{Int64,Int64} with 2 entries:
  2 => 4
  1 => 2

The deprecation will match the deprecation in:

julia> [1=>2, 2=>4]

WARNING: deprecated syntax "[a=>b, ...]" at REPL[53]:1.
Use "Dict(a=>b, ...)" instead.
Dict{Int64,Int64} with 2 entries:
  2 => 4
  1 => 2

[blakejohnson]

This could use an entry in NEWS.md

[tkelman]

fc3cbc9