still in work: stack-operations for push and pop for variables
added a symbol table and changed the variable operations to be local
fixed typo in push_into
Delete stack_variables.txt

fixed variables
added underflow_assert to pop_into
pleasing hlint
Merge branch 'master' into intertarget-code
#123: First attempt at defining a stack element struct.
#123: Fix typo in type name
Logically order types in stack.ll
Issue #123: Apparently structs do not need to be packed in our case.
#123: Document stack_element.
Issue #123: Explain that struct.stack_elem is misnamed.
#123: Implement stack_element_new().
Issue #123: String pushing for new linked stack, part 1
Issue 123: Use the new push functions.
Issue #123: Increment stack size when pushing.
#123: Fix stack_get_size().
#123: Implement new-style string pop.
#123: Use the new-style string pop.