Optimize memory handling (#1963)

* optimize ram block handling

Save the last element of the ram_list. This allows to
faster find where to add new elements when they are not
bigger then page size.

* save ram_list freed

this keeps the optimization for find_ram_offset() intact after snapshot
restore.

* cow only clear the tlb of affected pages

* update flatview when possible

Building each flatview new when the memory has changed is quite
expensive when many MemoryRegions are used. This is an issue when using
snapshots.

* update benchmark for new api

* save flatview in context

this avoids rebuilding the flatview when restore a context.

* init context flatview with zero

* address_space_dispatch_clear remove subpage with higher priority

* docutemnt the options for UC_CTL_CONTEXT_MODE

Specialy stress that with UC_CTL_CONTEXT_MEMORY it is not possible to
use the context with a different unicorn object.

symbols.sh

@@ -54,6 +54,7 @@ resume_all_vcpus \
 vm_start \
 address_space_dispatch_compact \
 flatview_translate \
+flatview_copy \
 address_space_translate_for_iotlb \
 qemu_get_cpu \
 cpu_address_space_init \
@@ -90,6 +91,7 @@ cpu_check_watchpoint \
 iotlb_to_section \
 address_space_dispatch_new \
 address_space_dispatch_free \
+address_space_dispatch_clear \
 flatview_read_continue \
 address_space_read_full \
 address_space_write \