Arbitrary Precision Integer Library

Written for educational purposes only. Not fully featured and unoptimised.

Supports the following operations:
 - add, subtract, multiply, divide (euclidean)
 - comparison operators (eq, lt, gt, lte, gte, neg)
 - pow, isqrt

Build and run unit tests:

```
$ cd src
$ make
etags *.[ch]
./arbint-test
PASSED : test_AI_Greater
PASSED : test_ai_add_unsigned_with_lshift
PASSED : test_ai_div_unsigned_by_subtraction__zero_result
PASSED : test_ai_div_unsigned_by_subtraction__even_divide
PASSED : test_AI_FromString_ToString__short_string
PASSED : test_AI_FromString_ToString__long_string
PASSED : test_addition__overflow
PASSED : test_addition__basic
PASSED : test_subtraction__basic
PASSED : test_from_string__basic
PASSED : test_to_string__dec_regression
PASSED : test_multiplication__basic
PASSED : test_division__simple
PASSED : test_string__base
PASSED : test_setbit__basic
PASSED : test_bititer__basic
PASSED : test_bititer__long
PASSED : test_hsb__1
PASSED : test_hsb__2
PASSED : test_hsb__4
PASSED : test_pow_by_pow2__1
PASSED : test_pow_by_pow2__2
PASSED : test_pow__two
PASSED : test_pow__three
PASSED : test_pow__seven
PASSED : test_string__speed_dec100
PASSED : test_string__speed_hex_wide
PASSED : test_string__speed_dec
PASSED : test_isqrt__small
PASSED : test_isqrt__large
PASSED : test_isqrt__very_large
PASSED : test_isqrt__very_large2
PASSED : test_isqrt__negative
```