~tim/scheme-vm

ref: c5110102ce2df2d78a0c87f293edf20f5a3984e7 scheme-vm/vm d---------
Add string->symbol function
Add symbol->string and symbol=? functions
Switch back to rust-peg and speed it up!

I was doing some stupid work repeatedly every time atom() was called.
Computing that work ahead-of-time puts rust-peg back on par with pest,
and I like rust-peg much better!
Remove parslet
Clarify purpose of this code
Store line and column number for better errors
Improve debugging of Unspecified instance
Add continuations and call/cc
Show a helpful error if a call site is invalid
Rearrange vm opcodes

* Rename ops to better match how they are used.
* Arrange opcodes alphabetically because I'm tired of
  agonizing over their ordering :-)
Build Char from stack value
Don't delete value from heap when doing RAW instruction
Remove CMP_EQV from vm -- implemented in scheme now
Move <= and >= into scheme and out of VM
Remove CMP_EQ_NUM vm instruction
Improve error output when stack overflows
Fix TCE when using apply func
Update code style
Implement equivalence semantics for empty lists and pairs
Fix spec compliance of different equivalence predicates
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