elevated ZSTD_getErrorCode() to stable status

suggested by @Eugeny1

copy "zstd_errors.h" alongside "zstd.h" to pass compilation test.

answering #4183

Block splitter control parameter

from ZSTD_c_useBlockSplitter to ZSTD_c_splitAfterSequences.

suggested by @terrelln

test both that the new parameter works as intended,
and that the over-split protection works as intended

not yet exposed to the interface.

Also: renames `useBlockSplitter` to `postBlockSplitter`
to better qualify the difference between the 2 settings.

Add fastest block-splitter variant

less precise but still suitable for `fast` strategy.

Added faster block splitter variants for levels 3-7

Block splitter

first block is no longer splitted since adding the @savings over-split protection

instead of just for blind split.

This is in anticipation of adversarial input,
that would intentionally target the sampling pattern of the split detector.

Note that, even without this protection, splitting can never expand beyond ZSTD_COMPRESSBOUND(),
because this upper limit uses a 1KB block size worst case scenario,
and splitting never creates blocks thath small.

The protection is more to ensure that data is not expanded by more than 3-bytes per 128 KB full block,
which is a much stricter limit.

issue reported by @terrelln

this helps make the streaming behavior more consistent,
since it does no longer depend on having more data presented on the input.

suggested by @terrelln

so that it can be stored using standard alignment requirement (sizeof(void*)).

Distance function still requires 64-bit signed multiplication though,
so it won't change the issue regarding the bug in ubsan for clang 32-bit on github ci.

strict C90 compliance test

due to integration of `sample5` strategy, leading to better compression ratios on a range of levels

detected by @felixhandte

following a discussion with @felixhandte

suggested by @terrelln

suggested by @terrelln