removes superfluous UCX_VERSION macro

fixes UCX_FOREACH documentation

fixed misaligned memory access in test_ucx_buffer_write

fixed further usages of SIZE_MAX

removed dependency to stdint.h in stack header

added casts for mallocs in AVL implementation (to satisfy c++ compiler)

removed some implementation details from documentation + fixed ucx_buffer_write doc

added sstrstr() function + improved sstrsplit variants by using sstrprefix()

added important test case to sstrsplit and fixed bug with consecutively occurring delimiters + fixed build system not linking test binary with newest library build

fixed name of ucx_list_free_content()

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