Support for SIMD operations for RGG1555

Description

Up to 3x the performance for RGG1555 with SIMD operations. The current implementation is hardcoded for 16 bytes at a time (128 bits). Still requires some testing and validation for AVX2 and AVX512.

Fixes #45 (closed)

benches/color.rs

-use boytacean::color::{rgb888_to_rgb1555_scalar, rgb888_to_rgb1555_simd};
+use boytacean::color::rgb888_to_rgb1555_scalar;
 
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
@@ -27,6 +27,7 @@ fn benchmark_rgb_conversion(c: &mut Criterion) {
 
     #[cfg(feature = "simd")]
     {
+        use boytacean::color::rgb888_to_rgb1555_simd;
         c.bench_function("rgb888_to_rgb1555_simd", |b| {
             b.iter(|| {
                 rgb888_to_rgb1555_simd(