Fix corrupted Vec creation in util::read_spir_v_file

[acgaudette]

I was debugging some strange memory problems and noticed that jemalloc would regularly freak out when Rust drops the buffer containing my shader bytecode. I traced it to this method.

Using Vec::from_raw_parts unfortunately can corrupt the data on the heap unless the new and old types share the same size and alignment.

https://doc.rust-lang.org/std/vec/struct.Vec.html#method.from_raw_parts

Instead, I used slice::from_raw_parts and to_vec() which is an in-place call (no copies) and should not incur an additional performance hit. The buffers are now dropped safely on my end.

Possibly related to #20--my issue has disappeared since this fix.

[acgaudette]

Note: after further testing, I determined this is unrelated to #20.

[acgaudette]

Replaced with #22