Add loading model param directly from memory.

[lygstate]

Conflicts:

src/net.cpp

src/paramdict.cpp

Conflicts:

src/net.cpp

[nihui]

only binary format memory loading is supported by design
If you want to embed your graph and model weight into code, stick to binary format
otherwise you will get larger binary, and all plain string literals are visible inside your binary ;)

load from	XYZ.param	XYZ.param.bin	XYZ.bin
file	y	y	y
memory	n	y	y