Compile `pleco` with stable Rust

[sfleischman105]

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

• const_fn (rust-lang/rust#24111)
• const_indexing (rust-lang/rust#29947)
• trusted_len (rust-lang/rust#37572)
• integer_atomics (rust-lang/rust#32976)
• allocator_api (rust-lang/rust#32838)
• cfg_target_feature
• target_feature
• stdsimd
• fused

For benchmarking:

• test (Switched to Criterion with Switch to Criterion for Benchmarking. #92 )

Dependency stabilization

• lazy_static nightly (no current tracking issue)

[sfleischman105]

Note, we should replace the use of core_intrinsics with something that will stabilize.

[sfleischman105]

This should be closed with the next release of pleco, 0.5.0.

[sfleischman105]

Fixed with PR #121, And pushed released with version #123 .