Revised the Index handling code.

Revised the Index handling code.

This breaks API and ABI but most apps are not affected
since most apps don't use this part of the API. You will
get a compile error if you are using anything that got
broken.

Summary of changes:

  - Ability to store Stream Flags, which are needed
    for random-access reading in multi-Stream files.

  - Separate function to set size of Stream Padding.

  - Iterator structure makes it possible to read the same
    lzma_index from multiple threads at the same time.

  - A lot faster code to locate Blocks.

  - Removed lzma_index_equal() without adding anything
    to replace it. I don't know what it should do exactly
    with the new features and what actually needs this
    function in the first place other than test_index.c,
    which now has its own code to compare lzma_indexes.