Speed improvement to lw_expr Instead of using a singly linked list and doing the slow append algorithm when adding an operand to an expression (scan from the start to find the end), now maintain a tail pointer. Also maintain a previous pointer in each entry. Benchmarking suggests this yields a rougly 30% improvement in runtime. Also refactor some of the code in lw_expr.c to make it somewhat clearer to understand. For some values of clearer and understand.

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