1) Improve the interface for constructing ForLoops  2) Remove the legacy elements of codegen from BlockStmt
Merge pull request #673 from noakesmichael/loopWrap-3

Minor changes for While/For loops

1) Add new constructors to ForLoop to improve the interface to builder code

2) Remove the For/While loop codegen code from BlockStmt

Compiled: clang std  on Mac,  gcc std/llvm on Linux
Testing: full paratest with STD --verify and LLVM --verify
Review: None