Commits on Jul 6, 2019

1. Fast-track @threads when nthreads() == 1 …

   This avoids overhead when threading is disabled, Example benchmark:
   
   ```
   using BenchmarkTools, Base.Threads, Test
   
   function func(val, N)
       sums = [0*(1 .^ val) for thread_idx in 1:nthreads()]
       for idx in 1:N
           sums[threadid()] += idx.^val
       end
       return sum(sums)
   end
   
   function func_threaded(val, N)
       sums = [0*(1 .^ val) for thread_idx in 1:nthreads()]
       @threads for idx in 1:N
           sums[threadid()] += idx.^val
       end
       return sum(sums)
   end
   
   @test func(2.0, 1<<10) == func_threaded(2.0, 1<<10)
   
   @show @benchmark func(2.0, 1<<10)
   @show @benchmark func_threaded(2.0, 1<<10)
   ```
   
   Running the benchmarks as:
   ```
   for JULIA in julia-master ./julia; do
       for T in 1 2; do
           echo "$JULIA with $T threads:"
           JULIA_NUM_THREADS=$T $JULIA speedtest.jl
       done
   done
   ```
   
   Before this commit:
   ```
   julia-master with 1 threads:
   julia-master with 2 threads:
   ```
   
   After this commit:
   ```
   ./julia with 1 threads:
   ./julia with 2 threads:
   ```

   

   staticfloat committed Jul 6, 2019
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