Run line through qalc on enter

Neat!  Currency conversion, math, cm to inches, things!

Also I was struggling with this so much and in the end the fix was to
put `[]const u8` in the signature instead of `?[*:0]u8`.  No idea why, I
thought I had already tried that, but that did the trick.  Well.

zig/sdl/hello_sdl.zig

@@ -151,7 +151,19 @@ pub fn main() !void {
    }
}
fn runCommand(cmd: ?[*:0]u8) ?[*:0]u8 {
    c.SDL_Log("cmd is '%s'", cmd);
    return "unknown command";
fn runCommand(cmd: []const u8, allocator: *std.mem.Allocator) ?[*:0]u8 {
    const argv = &[_][]const u8{ "/usr/bin/qalc", "-terse", cmd };
    const result = std.ChildProcess.exec(.{ .allocator = allocator, .argv = argv }) catch |err| return "oopsie";
    const buf = allocator.allocSentinel(u8, 100, 0) catch |err| return "alloc";
    std.mem.copy(u8, buf, result.stdout);
    var i: usize = result.stdout.len;
    while (i < buf.len) : (i += 1) {
        buf[i] = ' ';
    }
    buf[buf.len - 1] = 0;
    defer {
        allocator.free(result.stdout);
        allocator.free(result.stderr);
    }
    return buf;
}