Reference interpreter panics on let-bound values

[robdockins]

Cryptol> let x = zero:[8]
Cryptol> x@0
False
Cryptol> :eval x@0
cryptol: You have encountered a bug in Cryptol's implementation.
*** Please create an issue at https://github.com/GaloisInc/cryptol/issues

%< ---------------------------------------------------
  Revision:  8ed946ea75646f732e46c6917dd4e95a4d4e964e
  Branch:    master (uncommited files present)
  Location:  [Reference Evaluator]evalExpr
  Message:   var `<interactive>::x` is not defined
CallStack (from HasCallStack):
  panic, called at src/Cryptol/Utils/Panic.hs:21:9 in cryptol-2.10.0.99-inplace:Cryptol.Utils.Panic
  panic, called at src/Cryptol/Eval/Reference.lhs:1689:19 in cryptol-2.10.0.99-inplace:Cryptol.Eval.Reference
%< ---------------------------------------------------

I think it's probably fine to not support REPL let bindings for :eval, but we shouldn't panic.