feat: added a few algorithms that will be used for rewriting RSA algorithm that currently fails the tests

[tjgurwara99]

Closes #268

Added a few algorithms that will be useful for me to completely rewrite the RSA algorithm from scratch which is currently failing.

The changes I have made are as follows:

1. The Gcd algorithm in math/gcd package - I changed the name of this to Recursive because when we call it from the outside, it will appear as gcd.Recursive which is easier to read (when reviewing) than the gcd.GcdRecursive.
2. Added another Gcd Algorithm which works iteratively within the same package math/gcd called Iteratively . I've added a benchmark for it as well so people can see why this is faster than the Recursive one.
3. Added an Lcm algorithm that works using the gcd.Iterative in its parent directory - notice the import call.
4. Added Miller Rabin Primality Test, which is used inside the RSA algorithm for testing probable primes. This is almost identical to the one I wrote in the TheAlgorithms/C-Plus-Plus repository but of course written in Go.
5. Changed the name of the package of primecheck to prime for better import organisation.

Apologies for a big PR but this is so that the repository is better organised - instead of rewriting the same algorithms multiple times, it is better to just include it in the imports unless there is an improvement to the already existing one (which can be shown using the benchmarks and its b.ReportAlloc() method). This way the primary concern of the algorithm can be separated from the nitty-gritty details of other small algorithms/functions that are within the algorithm that is being added.