(float) fix some rounding errors when showing as str

[killerswan]

This seems to fix issue #1876, and some of the superficial parts of
issue #1375. The #fmt macro and the to_str functions will round,
rather than truncate, floats as strings.

Other issues remain, and I wrote more code here than intended, but the
following should pass now.

fn x() {
   assert "3.1416"      == #fmt["%.4f", 3.14159];
   assert "3"           == #fmt["%.0f", 3.14159];
   assert "99"          == #fmt["%.0f", 98.5];
   assert "7.0000"      == #fmt["%.4f", 6.999999999];
   assert "3.141590000" == #fmt["%.9f", 3.14159];
}

[brson]

Thanks!