interpret: refactor projection handling code

Moves our projection handling code into a common file, and avoids the use of a
general mplace-based fallback function by have more specialized implementations.

mplace_index (and the other slice-related functions) could be more efficient by
copy-pasting the body of operand_index. Or we could do some trait magic to share
the code between them. But for now this is probably fine.

compiler/rustc_const_eval/src/interpret/mod.rs

@@ -9,6 +9,7 @@ mod memory;
 mod operand;
 mod operator;
 mod place;
+mod projection;
 mod step;
 mod terminator;
 mod traits;