Move Bit_allocator from base-nova to base

Change the template parameter for Bit_allocator, and Bit_array. Instead of
assigning words to be used by the bit array, you can now tell the count of
items that shall be used.

Moreover, some dead code, previously using the Bit_allocator, was removed.

Related to #1024
thread: rearrange thread context management

Use a bit allocator for the allocation management of thread contexts,
instead of holding allocation information within the Thread_base objects,
which lead to race conditions in the past.

Moreover, extend the Thread_base class interface with the ability to
to add additional stacks to a thread, and associate the context they're
located in with the corresponding Thread_base object. Additional stacks
can be used to do user-level scheduling with stack switching, without breaking
Genode's API.

Fixes #1024
Fixes #1036