Manage breakpoints with red-black tree

[RinHizakura]

This patch enables rv32emu to set more than one breakpoint (should be infinite if we assume our memory is always enough to
create a new tree node).

Note that it's still bad to query the PC address in every instruction step. This patch only aims to fix the limitation of the breakpoint number in a short time. We may make use of the emulator's trap mechanism to avoid frequently querying the RB tree

1. Replace the instruction of the breakpoint's address with a breakpoint instruction
2. Query the RB tree when the emulator hit the trap when executing such instruction