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

This avoids overhead when threading is disabled, Example benchmark:

```
using BenchmarkTools, Base.Threads

function func(val)
    local sum = 0
    for idx in 1:100
        sum += idx.^val
    end
    return sum
end

function func_threaded(val)
    local sum = 0
    @threads for idx in 1:100
        sum += idx.^val
    end
    return sum
end

@show @benchmark func(2.0)
@show @benchmark func_threaded(2.0)
```

Before change:
```
@benchmark(func(2.0)) = Trial(94.464 ns)
@benchmark(func_threaded(2.0)) = Trial(8.228 μs)
```

After change:
```
@benchmark(func(2.0)) = Trial(93.766 ns)
@benchmark(func_threaded(2.0)) = Trial(93.765 ns)
```

Author: staticfloat

base/threadingconstructs.jl

@@ -97,6 +97,10 @@ macro threads(args...)
         throw(ArgumentError("need an expression argument to @threads"))
     end
     if ex.head === :for
+        # If we are only running with one thread, just run the for loop directly
+        if nthreads() == 1
+            return esc(ex)
+        end
         return _threadsfor(ex.args[1], ex.args[2])
     else
         throw(ArgumentError("unrecognized argument to @threads"))