* Fix crash issue during load_model

* Fix crash issue during load_model 2nd part

A project using src/CMakeLists.txt directly does not want
to install NCNN library and headers. This new option makes it
optional (default to true).

* requantize layer arm64-v8a neon implement

* convdw3x3s1 arm64-v8a neon implement

* convdw3x3s2 arm64-v8a neon implement

* conv1x1s1 arm64-v8a is optimized by neon assembly

* conv sgemm int8 optimized with neon assembly,kernel transform is offline

* conv conv winograd int8 optimized with neon assembly,fix ci build failed

* conv3x3s2 int8 arm64-v8a optimized with neon assembly,remove old codes.

* add the armv7a conv3x3s1 implement without overflow,remove old codes

* fix the bug of conv3x3s2 packed int8

* new int8 implement,weight quant by perchanel,better accuracy~

* fix the bug of conv3x3s1 packed int8 neon

* add the naive c fp32 and int8 winograd F(2,3)

* add the neon intrinsic int8 winograd F(2,3)

* optimize the armv7a int8 winograd F(2,3) with neon assembly

* optimize the armv7a int8 winograd F(2,3) input transform with assembly.

* add the requantize layer and int8 relu implement.

* add graph optimize conv1x1s2 -> conv1x1s1,begin optimize int8 aarch64.

* fix int8 bugs

* add the c naive im2col with sgemm

* add aarch64 int8 winograd f23, conv3x3s2 naive implement

* add the int8 sgemm conv7x7s2 on x86/armv7a platform

* optimize the int8 sgemm by neon intrinsic and packed kernel

* optimize the int8 sgemm with packed data

* optimize the int8 sgemm with armv7a neon assembly

* add the int8 sgemm on arm64-v8a platform

* perpare to merge latest codes from master

* add the int8 param files

* In the Class Net,add the fuse_network method