WIP: Fix typo in util.c

Fix null pointer addition

[cmake][tests] Add zstreamtest executable

Update .gitignore

Easy: Update Comment on `ZSTD_initDStream()`

Fixing hashLog error 2^26=64Mib

Fix Appveyor failure

There must be mtim*e*

Optimized loop bounds to allow the compiler to unroll the loop.

RLE test and re-enable RLE in main compression loop

shared library: rename import library with .dll.a extension

mort of open source project are using this extension for the import library.
The Win32 linker is supporting this extension, see
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/win32.html
section "direct linking to a dll"

This has no measurable impact on large files but improves small file
decompression by ~1-2% for 10kB, benchmarked with:

head -c 10000 silesia.tar > /tmp/test
make CC=/usr/local/bin/clang-9 BUILD_STATIC=1 && ./lzbench -ezstd -t1,5 /tmp/test

Superblocks rebased for merge