better no_std support

[alxiong]

• use core/alloc instead of std

[dot-asm]

Just in case, I'd be hesitant to go "all-in" for no_std. If we go there, it would be more than appropriate to have it as a compile-time option. I assume that this is about embedded systems, so we're looking specifically at the most basic functionality, right? With no_std being an option it would be possible to mask some of the advanced functionality that would be in the way.

[alxiong]

Just in case, I'd be hesitant to go "all-in" for no_std. If we go there, it would be more than appropriate to have it as a compile-time option.

yes, I plan to use feature flag to turn on/off no_std instead of enforcing #![no_std]. was occupied by other work, will continue later.

[nazar-pc]

Standard feature flag-based solution in #150

[dot-asm]

There is #153 in works.