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];
}
```
a7359f5b3b