Compile `pleco_engine` with stable Rust

[sfleischman105]

The following features need to be stabilized before pleco_engine can compile on stable:

• [ ] const_fn (rust-lang/rust#24111)
• [x] const_indexing (rust-lang/rust#29947)
• [ ] fused (rust-lang/rust#35602)
• [ ] integer_atomics (rust-lang/rust#32976)
• [ ] allocator_api (rust-lang/rust#32838)
• [ ] trusted_len (rust-lang/rust#32838)
• [ ] ptr_internals

For benchmarking:

• [x] test (Switched to Criterion with #92 )

Dependency stabilization

• [ ] lazy_static nightly (no current tracking issue)
• [ ] pleco needs to be usable on stable as defined by #76 .

[looak]

error[E0554]: #![feature] may not be used on the stable release channel
  --> C:\Users\alexa\.cargo\registry\src\github.com-1ecc6299db9ec823\prefetch-0.2.0\src\lib.rs:18:1
   |
18 | #![feature(link_llvm_intrinsics)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error 

So what version or branch of rust should one be using?

[sfleischman105]

@looak currently, nightly rust is required to use pleco. I'll double check though to make sure it still compiles on nightly.

I'll look into having a 'nightly' feature flag for pleco. I think most of the unstable features aren't exactly needed, but are there for mostly speed / convenience.