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.

[ojwb]

This type incompatibility comes from using an old version of Snowball's rust runtime with rust code generated by a newer version of the Snowball compiler.

i_pV comes from a Snowball integer variable which can have negative values and so really does need to be a signed type, but the original rust output and runtime used an unsigned type. That was fixed in 2018 (by snowballstem/snowball@64419d4) - this PR is trying to effectively revert the effects of that on generated rust code.

The best fix is a wholesale upgrade to the latest Snowball compiler and runtime, but if you really want/need to patch older stuff, make limit_backward, limit, cursor, etc i32 instead.