ntheory: get rid of the static_casts
Use integer() instead of calling "new" directly
Use int with probab_prime_p instead of Integer
ntheory: move implementation of fns to .cpp file

This way the header file is small and compiles faster. Only if it turns out
that using "inline" is faster for some particular algorithm, should we use
inline functions.
Use `integer()` instead of `Integer`
Implemented 'gcd_ext()'
Implement `mod_inverse()`
Use `add()` and `mul()`
Implement `isqrt()`
Remove unused include and import
Implement `iabs()`
Readd mistakingly untracked `test_integer.cpp`
Implement `i_nth_root()`
Implement `perfect_power()` and `perfect_square()`
Merge pull request #100 from certik/ntheory_improvements

Merged #100