Optimizations in `message_wire_format.h`:
Optimizations in `message_wire_format.h`:

- Compute varint length only if that can be done at compile time: replace
  `RIEGELI_IS_CONSTANT(data < 0x80)` with
  `RIEGELI_IS_CONSTANT(data < 0x80) && data < 0x80`. If `data` is known to be
  larger, the length is not known at compile time.

- Check whether tag length is known at compile time, instead of always using
  its true length.

- Use `WriteVarint32()` instead of `WriteVarint64` in `WriteVarint32WithTag()`.

Also, remove `EncodeDouble()` which is no longer used.

PiperOrigin-RevId: 459169487

google/riegeli,google/riegeli,google/riegeli,google/riegeli