Improves decompression speed when using cold dictionary

 improve long-range decoder speed

by triggering the prefetching decoder path
(which used to be dedicated to long-range offsets only).

Figures on my laptop :
no content prefetch : ~300 MB/s (for reference)
full content prefetch : ~325 MB/s (before this patch)
new prefetch path : ~375 MB/s (after this patch)

The benchmark speed is already significant,
but another side-effect is that this version
prefetch less data into memory,
since it only prefetches what's needed, instead of the full dictionary.

This is supposed to help highly active environments
such as active databases,
that can't be properly measured in benchmark environment (too clean).

Also :
fixed the largeNbDict test program
which was working improperly when setting nbBlocks > nbFiles.

Add TSAN dictionary tests

one limit case can apparently be generated during fuzzer tests

as it was broken and no one noticed

on enwik9 at level 22 (which is almost a worst case scenario),
speed improves by +7% on my laptop (415 -> 445 MB/s)

Prevent deadlock on malloc() failure.

Allow creating debug builds with CMake

Only set the build type to Release if the user didn't select any.
See discussion in https://github.com/facebook/zstd/pull/1278#issuecomment-436327923
and below.

fixed T36302471

just add an assert() in ZSTD_insertBtAndGetAllMatches()

to express a condition on ll0 .
May help static analyzer as in #1397

created zstd_decompress_block module 

Fix building zstd-pgo

not sure why it only triggers now,
this code has been around for a while.

Introduced a new error code : dstBuffer_null,
I couldn't express anything even remotely similar with existing error codes set.

memcpy(NULL, src, 0) is undefined behavior.

refactor HUF_compress_internal for clarity 

and clean *.o so that linking does not fail from code
that is partially augmented with coverage generation instructions

otherwise, there might not be a ./zstd to run

for library and cli

to conform fb publication rules.

provide consistent spacing to enum field

Publish uncompressed, zstd, and gzipped packages

support decompressing an empty frame into NULL

changed workspace parameter convention
to always provide workspaceSize,
so that size can be explicitly checked.

Also, use more enum to make the meaning of some parameters more explicit.