Refactor to reduce dependancy on ios.c

Rationale:

    Unravel ios.c’s legacy role as debug io library for the runtime;
    vs current role as backend for iostream.jl.

Clean up historical debug printf functions:

    Replace calls to ios_printf(), ios_puts(), JL_PRINTF, JL_PUTS etc
    with jl_printf(), jl_puts()...

    Replace exit() with jl_exit() in repl.c, dump.c and init.c.

Make jl_printf() and jl_exit() safe in early initialisation.

Remove ios.c dependance from ast.c and builtins.c:

    src/flisp/flisp.c:

        New fl_load_system_image_str(char*, size_t)
        Hides detail of ios_static_buffer from ast.c
        ios_* no longer used in ast.c.

    src/builtins.c

        In jl_errorf(), replace ios_vprintf() with vasprintf()
        (following the precedent set by src/jl_uv.c:663).

Use free() not LLT_FREE() to clean up after vasprintf() in jl_uv.c:

    man vasprintf says “asprintf() and vasprintf() dynamically
                        allocate a new string with malloc(3)”

Refactor pairs of calls to jl_printf() & jl_exit(1) into calls to
jl_error[f](), which does the same thing during early init.

Dead code removal:

    jl_uv.c: jl_bufptr(), jl_ios_buf_base(), jl_putc(), jl_pututf8()
    sys.c: jl_ios_size()

New jl_safe_printf().

jl_safe_printf() uses a statically allocated 1000 byte buffer to
vnsprintf() a message then calls write(2) to send it to stderr.

jl_safe_printf() is intended to be used in places where there is the
potential for out of memory, corrupted stack, re-entrancy etc.

The current jl_safe_printf() implementation is not 100% safe in all
situations, but it should be pretty good most of the time, especially
if the formatting is limited to %d.

The safety of jl_safe_printf() could be improved later, but at least
for now the places were safety might matter are identified by its use.

jl_safe_printf() is used in:

 - sigdie_handler()
 - sigint_handler()
 - _exception_handler()
 - catch_exception_raise()
 - gdblookup()

Thanks to @vtjnash for assistance and feedback with these changes.