Introduce ParamForLoop as a thin veneer on BlockStmt
ParamForLoop: Refactor the constructor
Convert ParamForLoop to inherit from LoopStmt
Merge pull request #989 from noakesmichael/param-for-loop-0

Introduce the statement ParamForLoop

This is the remaining looping AST node that relies on loop state within BlockStmt.  Introduce a new AST node for this type of loop that inherits directly from LoopStmt.

1) Introduce a simple version of ParamForLoop as a direct decedent of BlockStmt

2) Add a "factory" method on the class ParamForLoop to support the parser/builder.

3) Migrate ParamForLoop from a direct dependence on BlockStmt to a direct dependence on LoopStmt.

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest --verify
Review: none