Bitarray

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* add optional `buffer` argument to `bitarray()` to import the buffer of
another object, 141, 146, see also: [buffer protocol](buffer.rst)
* update `.buffer_info()` to include: a read-only flag, an imported buffer
flag, and the number of buffer exports
* add optional start and stop arguments to `util.rindex()`
* add [memory-mapped file](../examples/mmapped-file.py) example
* ignore underscore (`_`) in string input, e.g. `bitarray('1100_0111')`
* add missing type hinting for new `.bytereverse()` arguments
* fix `.extend()` type annotations, 145
* avoid `.reverse()` using temporary memory
* make `.unpack()`, `util.serialize()`, `util.vl_encode()`
and `.__reduce__()` more memory efficient
* add and improve tests

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* speedup `find_bit()` and `find_last()` using uint64 checking, this means
a speedup for `.find()`, `.index()`, `.search()` and `util.rindex()`
* add optional start and stop arguments to `.bytereverse()`
* add example to illustrate how
[unaligned copying](../examples/copy_n.py) works internally
* add documentation
* add tests

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* use shift operations to speedup all unaligned copy operations, 142
* expose functionality to Python level only in debug mode for testing
* add and improve tests

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* speedup `repeat()`, 136
* speedup shift operations, 139
* optimize slice assignment with negative step, e.g.: `a[::-1] = 1`
* add tests

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* speedup slice assignment, see 132 and 135
* speedup bitwise operations, 133
* optimize `getbit()` and `setbit()` in `bitarray.h`
* fix TypeError messages when bitarray or int (0, 1) are expected (bool
is a subclass of int)
* add and improve tests

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* improve documentation
* speedup `vl_encode()`
* `bitarray.h`: make `getbit()` always an (inline) function
* add assertions in C code