0.1.6

Updated pleco to 0.5.0 and pleco_engine to 0.1.6

Mostly little fixes, but most importantly, Pleco now compiles on stable Rust!

pleco changes:

• Now compiles on stable!!
• Added nightly feature for advanced optimizations on nightly toolchain.
• Modified move_list::MAX_MOVES to adjust for cache size based on target pointer width.
• Removed prefetch dependency and added a new prefetch.
• Improved PRNG::singular_bit()'s randomness (Better distribution of bits).
• Allocation now depends on Alloc, not GlobalAlloc.
• popcount now uses a table lookup by default.
  -bit_scan_forward now does an unchecked lookup on the De Bruijn table
• minimax cleanup.

pleco_engine changes:

• Modified prefetch instructions to use pleco::prefetch_write
• Refactored some methods in threadpool.rs