Doesn't Compile due to const_fn feature having been removed

[duganc]

It looks like compilation (on Nightly, release) is failing due to:

error[E0557]: feature has been removed
  --> pleco_engine\src/lib.rs:24:12
   |
24 | #![feature(const_fn)]
   |            ^^^^^^^^ feature has been removed
   |
   = note: split into finer-grained feature gates

[jbgriesner]

PR done: #138

[chase-manning]

@duganc @jbgriesner This has been fixed now 😄