Port derefs.nim in Nimony to Nim 2

[demotomohiro]

Port derefs.nim in Nimony to Nim 2 and remove hacks from Nim 2.

Steps:

• Borrow / Mutability checks

  • [ ] locations
  • [ ] return statements
  • [ ] assign statements
  • [ ] yield statements
  • [ ] call expressions

• Add hidden deref/addr

  • [ ] location
  • [ ] call expression
  • [ ] deref

• [ ] Check for dangerous locations

[Araq]

The "Check for dangerous locations" is required by polymorphic accessors which Nim 2.0 does not have. But it's a good idea to add them too so that Nim 2.0 can get polymorphic accessors.