Perl: Decoder refactoring
Perl: Fix incremental parsing tests

Broken in recent LZ4 change to the test file.
Perl: Do not bother copying the header after decompression
Perl: Further decoder simplification
Perl: Micro-optimization in encoder

Just avoiding a branch in the encoder cleanup.
Probably saved half a nanosecond per encode only even if branch
prediction were not a thing.
Perl: Cannot really use tmp_buf here

We would overwrite the location of the allocated buffer.
That in turn means we would have to save that one. At which
point the whole thing becomes a bit of a farce. So let us not
do that.