Optimize fibril creation

[Martin Decky]

The creation of new fibrils and allocating stacks for them for every single IPC notification (e.g. interrupt notification) puts a lot of stress on the system. It is observed to even cause PS/2 protocol desynchronization on very slow machines (e.g. QEMU).

There are several ways how this issue can be mitigated. This ticket provides an overview of them:

• Optimizing the new bitmap frame allocator (especially the bitmap search itself, which is currently very suboptimal).
• Optimizing the page mapping manipulation by caching the intermediate levels of the page table.
• Implementing a fibril pool that would avoid the need to create and discard all the precious resources for each single notification.