This way we can support fewer build configurations. Also, remove
all undefined behavior due to use of memcpy with NULL pointers. GCC
was exploiting this to remove NULL checks in some builds.
This lets us build a smaller binary. The minimal tested
binary on x86-64 (with -Os, -s, and all options that shrink binary size
turned on) is about 240 kB.
Allow overriding functions in the core libray to provide better
functionality on startup. Used to include our getline function in
the repl but use a simpler version in the core library.
This should speed up start time and reduce malloc/free
usage to about 15% of what is what previously for startup.
The current cost is slightly larger binary as the representaion
of the image is currently less compact than source code.
