Simplify IO_CACHE by removing current_pos and end_pos as self-references

These self references were previously used to avoid having to check the
IO_CACHE's type. However, a benchmark shows that on x86 5930k stock,
the type comparison is marginally faster than the double pointer dereference.
For 40 billion my_b_tell calls, the difference is .1 seconds in favor of performing the
type check. (Basically there is no measurable difference)

To prevent bugs from copying the structure using the equals(=) operator,
and having to do the bookkeeping manually, remove these "convenience"
variables.

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