propagate the error of missing symbol back to ks_asm(). this adds new error code KS_ERR_ASM_SYMBOL_NOTFOUND. fix issue #61

Uses new / delete like in mrexodia's patch, deletes things allocated
in ks_asm properly, clean up properly in ks_close. Removed 2 deletes
from the MCObjectStreamer destructor that we re-use in a second run
of ks_asm on the same keystone instance.

Valgrind shows no invalid reads and no memory leaks in keystone after
running the regression tests.

Besides fixing `ldr Rd,=label` on ARM, this also fixes defining addresses
of symbols as literals (`.int label1`) on all platforms.