Fixed Context struct to have usize instead of i32

[wdoppenberg]

I kept getting an error as follows:

error[E0308]: mismatched types
   --> src/snowball/algorithms/armenian.rs:352:26
    |
352 |     env.limit_backward = context.i_pV;
    |     ------------------   ^^^^^^^^^^^^ expected `usize`, found `i32`
    |     |
    |     expected due to the type of this binding

For more information about this error, try `rustc --explain E0308`.

This was easily fixed by changing the Context struct's fields to be usize.

• ran cargo test

Furthermore, I added a basic GH Actions workflow to run cargo test.