switch to nightly and enable portable_simd feature

i really did not want to use nightly, but i think it is worth it for
portable_simd which is still nightly-only (and i'm guessing will be
for a long while still). i was not particularly enthusiastic about any
of the alternative libraries that exist for stable rust simd, as it
always seemed like something small was missing from what i knew i would
want longer term.

ggdt/src/lib.rs

@@ -1,3 +1,5 @@
+#![feature(portable_simd)]
+
 pub mod audio;
 pub mod base;
 pub mod entities;

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly"