[btcsuite/btcd] 7411e6: txscript: Unexport Stack type.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcd
  Commit: 7411e65b1e8d4e4c6da198590ad627bfe253173c
      
https://github.com/btcsuite/btcd/commit/7411e65b1e8d4e4c6da198590ad627bfe253173c
  Author: Dave Collins <da...@conformal.com>
  Date:   2015-04-25 (Sat, 25 Apr 2015)

  Changed paths:
    M txscript/engine.go
    M txscript/stack.go
    M txscript/stack_test.go

  Log Message:
  -----------
  txscript: Unexport Stack type.

This commit unexports the Stack type since it is only intended to be
used internally during script execution.  Further, the engine exposes
the {G,S}etStack and {G,S}etAltStack functions which return the items as
a slice of byte slices ([][]byte) for caller access while stepping.