Update base/ascii.jl

```
julia> s=randstring(10000).data;

julia> memchr(s,'C',1)
178

julia> function f(s,c,i)
         for j in i:length(s)
           if s[j]==c
             return j
           end
         end
         return 0
       end

julia> f(s,'C',1)
178

julia> benchmark(()->memchr(s,'C',1),()->f(s,'C',1),"cmp","memchr","for",1000)
memchr 1.4216811568648726e-6, 1.6298568674055228e-8
for 9.931015133393878e-7, 2.4488361237383283e-8
memchr wins to for on the 12.242798353909464% of runnings
for wins to memchr on the 53.18930041152263% of runnings
Ties: 34.5679012345679%

```

I read about memchr comparing more than one byte per cycle (value depends on 32 o 64 bits computer)[1]. Maybe in my memchr is more efficient, but... Looks like a simple for can be a little more faster on Julia. Maybe is good if you test the same too.
About other topic on [1]... In Julia do 'a'|32 is a little more slower than do 'a'-32



[1] http://www.codemaestro.com/articles/21

P.S.: The benchmark function:
```
function benchmark(f::Function, f2::Function, category::String, name::String, name2::String, N::Int)
  choosed = rand(2N) .<= .5
  N1= sum(choosed)
  N2= 2N - N1
  times1 = zeros(N1)
  times2 = zeros(N2)
  f() # Call once to force JIT compilation
  f2()
  i=1
  j=1
  for itr in 1:2N
    gc()
    if choosed[itr]
      times1[i] = @elapsed f()
      i += 1
    else
      times2[j] = @elapsed f2()
      j += 1
    end
  end
  println("$name $(mean( times1 )), $(std( times1 )/sqrt(length(times1)))")
  println("$name2 $(mean( times2 )), $(std( times2 )/sqrt(length(times2)))")
  println("$name wins to $name2 on the $(N1 > N2 ? mean( times1[1:N2] .< times2 )*100 : mean( times1 .< times2[1:N1] )*100)% of runnings")
  println("$name2 wins to $name on the $(N1 > N2 ? mean( times1[1:N2] .> times2 )*100 : mean( times1 .> times2[1:N1] )*100)% of runnings")
  println("Ties: $(N1 > N2 ? mean( times1[1:N2] .== times2 )*100 : mean( times1 .== times2[1:N1] )*100)%")
  
end
```