handle equal directive. this fixes c-crashes/crash-12-x64-cannot-set-a-variable-that-has-already-been-used.c

handle error in applyFixup(). this fixes c-crashes/crash-23-x64-value-does-not-fit-in-the-fixup-field.c

x86: report unsupported instructions in encodeInstruction(). this fixes c-crashes/crash-24-x64-not-is16bitmemoperand.c

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

add error code KS_ERR_ASM_SYMBOL_REDEFINED to report redefined symbols. also updated all the bindings after this