Add stim.CliffordString (#910)

- Add `stim.CliffordString`
- Add `stim.CliffordString.__eq__`
- Add `stim.CliffordString.__ne__`
- Add `stim.CliffordString__imul__`
- Add `stim.CliffordString__init__`
- Add `stim.CliffordString.__len__`
- Add `stim.CliffordString.__mul__`
- Add `stim.CliffordString.__repr__`
- Add `stim.CliffordString.__setitem__`
- Add `stim.CliffordString.__getitem__`
- Add `stim.CliffordString.random`
- Add `stim.CliffordString.x_outputs`
- Add `stim.CliffordString.y_outputs`
- Add `stim.CliffordString.z_outputs`

C++ changes:
- Add `stim::simd_bits<>::clear_bits_past`
- Add `stim::simd_bits<>::preserving_resize`
- Add `stim::bitword<>::operator~`
- Add `stim::andnot`
- Add `stim::GateTypeWrapper`
- `stim.GateData` now binds to `stim::GateTypeWrapper` rather than
`stim::GateData` (a byte instead of many bytes)
- Ran autoformat (forced pointed location to RIGHT)
- Add `stim::CliffordString<>`

---------

Signed-off-by: Craig Gidney <craiggidney@google.com>

Author: Strilanc

.clang-format

@@ -11,6 +11,7 @@ AllowShortLoopsOnASingleLine: false
 AllowShortLambdasOnASingleLine: None
 BinPackArguments: false
 BinPackParameters: false
+PointerAlignment: Right
 
 IncludeCategories:
   - Regex:           '^<.*>$'