New step implementation

[ncannasse]

New step implementation allows to put pseudo-breakpoints at places of interest depending on the desired action, which is much more efficient than previous stepping that was executing and pausing each single cpu instruction.

However the following limitations are currently in place :

• exceptions : step when an exception is throw will handle it, whereas we would maybe like to step in the catch handler
• step in native : calls to native (C) functions are skip when doing step-in, as well as std library. As a result, a call to native calling back to Haxe cannot be step-in.

Waiting for feedback and reproducible cases to improve things further.