the only value which makes sense is `hashlog-1`
as it mimics the real memory usage.

fixed incorrect rowlog initialization

answers #2764

the variable has only very limited usage,
being only used once at the beginning of the block for prefetching only,
hence the error had no impact on compression ratio.

Allow user to specify memory limit for dictionary training

simplify SSE implementation of row_lazy match finder

Reduce tables to 8bit

This saves some 1.7Kb in rodata section (x86_64, zstd tool),
while assembler code stays the same except
the type of a few load/extend instructions.

Should not have negative performance implications.

To make code more expressive, add U8 and S8 typedefs

Stagger Stepping in Negative Levels

mostly for maintenance convenience.

Performance wise, there is very little change,
slightly faster for slog 3 & 4,
neutral or very slightly negative for slot 5 & 6.

I couldn't find a good way to spread `ip0` and `ip1` apart when we accelerate
due to incompressible inputs. (The methods I tried slowed things down quite a
bit.)

Since we aren't splaying ip0 and ip1 apart (which would be like `0_1_2_3_`, as
opposed to the `01__23__` we were actually doing), it's a big ambitious to
increment `step` by 2. Instead, let's increment it by 1, which has the benefit
sliiightly improving compression. Speed remains pretty much unchanged.

add test case

The position updates are rewritten from `ip[N] = ip[N-1] + step` to be
`ip[N] = ip[N-2] + step`. This lets us only deal with the asymmetric spacing
of gaps at setup and then we only have to keep a single `step` variable.

This seems to work quite well on GCC and Clang!

typo: Small spelling mistake in example

Just a couple of characters:
`main` -> `may`

x32 compatibility

This avoids an additional addition, at the cost of an additional variable.

This replicates the behavior of @terrelln's `ZSTD_fast` implementation. That
is, it always looks at adjacent pairs of positions, and only applies the
acceleration every other position. This produces a more fine-grained
acceleration.

Update changelog for v1.5.1

Reject Irregular Dictionary Files

updated xxHash to latest v0.8.1

no idea why visual + clang-cl + appveyor don't like them,
I've not been able to reproduce the issue locally,
but these static assert are very unlikely to deliver a useful signal,
I can't imagine a situation where they will be wrong,
and if they are, then a ton of other things will be broken way before reaching that point.