Implement a simple demangler.

The purpose of this demangler is to avoid crashes for any string.

- It does one pass and should avoid going past the end of the string.
- The code avoids recursion to minimize the amount of stack required.
- It cannot demangle all mangled names, but it should be able to work
  on nearly all names in normal stack traces.
- If the mangled name is too large, it will stop demangling and return
  as if the name is not a demangled name.

Test: Passes new unit tests.

Change-Id: I596f74a533c0e093d1517c6bd11cced07009d321