Pure double to string conversion

[Per Nordlöw]

Is there a way to convert a `double` to a `string` in a `pure` 
function?

I tried

@safe pure unittest
{
     import std.conv : to;
     const y = 3.14.to!string;
}

but it fails to compile as

`pure` function `foo.__unittest_L1_C12` cannot call impure 
function `std.conv.to!string.to!double.to`

Have anybody wrapped the corresponding C solution via, snprintf 
or gcvt in a high-level D wrapper?

Is assume `gcvt` is GNU only but that's ok with me. I assume 
`gcvt` is faster than `snprintf` in this case.