fixed buggy decompression

decompress was behaving incorrectly on certain inputs
because the return value of c_LZ4_decompress was
misinterpreted to represent the number of bytes used in result buffer
rather than the number of bytes read from the input compressed buffer
added ScopedTypeVariables extension so that I can annotate
the variable `r` in decompress to have type Int so that no warnings about defaulting r to Integer happen.
Merge pull request #6 from cartazio/master

fixed long standing decompression bug!!!