feat: implemented the string module

[RonnieSalomonsen]

Hi,

Ported the String Module. Please peer review.

cargo test --package yara-x --lib --features string-module -- modules::string --nocapture

   Compiling yara-x v0.1.0 (yara-x/yara-x)
    Finished test [unoptimized + debuginfo] target(s) in 5.95s
     Running unittests src/lib.rs (target/debug/deps/yara_x-ff5dd03397465a33)

running 1 test
test modules::string::tests::end2end ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 32 filtered out; finished in 0.22s