MAJOR REWRITE.

- stop passing Buf/BufEnd to every function (now member vars)
- internalize things that used to be in a header file that no one else
  included/needed.
- Remove defunct BCR_TRACE lines
- Standardize error handling with the PARSE_ERROR macro.
- Integrate ConstantReader.cpp and InstructionReader.cpp and reorgnize
  the definition order so that gcc has a chance at optimizing this module
- Standardize case and style of method names.
- Eliminate unneeded header files
- Prepare for Type != Value (bug122) change by splitting Types into their
  own data structures.
- Implement the BytecodeHandler interface calls.
- Provide default implementation of BytecodeHandler interface.

llvm-svn: 14494