Widen useful range of BitWriter's write_quniform()

The versions of write_quiniform() used by the arithmetic packer and the uncompressed bitwriter vary considerably.  For some reason, the BitWriter uses a much 'narrower' implementation that overflows with inputs of more than a few bits.

This patch replaces the BitWriter's implementation with one like in the arithmetic packer.  This allows its use in coding non-uniform tile header fields.

src/ec.rs

@@ -804,17 +804,17 @@ impl<W: io::Write> BCodeWriter for BitWriter<W, BigEndian> {
  }
  }
  fn write_quniform(&mut self, n: u16, v: u16) -> Result<(), std::io::Error> {
- /* Encodes a value v in [0, n-1] quasi-uniformly */
- if n <= 1 {
- return Ok(());
- };
- let l = 31 ^ ((n - 1) + 1).leading_zeros();
- let m = (1 << l) - n;
- if v < m {
- self.write(l - 1, v)
+ if n > 1 {
+ let l = msb(n as i32) as u8 + 1;
+ let m = (1 << l) - n;
+ if v < m {
+ self.write(l as u32 - 1, v)
+ } else {
+ self.write(l as u32 - 1, m + ((v - m) >> 1))?;
+ self.write(1, (v - m) & 1)
+ }
  } else {
- self.write(l - 1, m + ((v - m) >> 1))?;
- self.write_bit(((v - m) & 1) != 0)
+ Ok(())
  }
  }
  fn write_subexpfin(