Allow contract state to be read (but not written to) from pure code

[jjcnn]

It is sometimes necessary to perform a recursive check for the contents of a map.

An example is the Shogi contract, where the board is represented as a map, and where we need to check that when a piece moves from square A to square B, no intervening square is occupied by another piece.

This check needs to happen using recursion, which puts us in the pure part of Scilla where map reads can only happen on a copy of the board map, not directly on the map in the contract state.

It would be more convenient and probably also more efficient to be able to read from the map during the recursive traversal, and in general read from the contract state in the pure part of Scilla.

[vaivaswatha]

@jjcnn is this also related to #254 ?