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.

include/unicorn/unicorn.h

@@ -1018,6 +1018,16 @@ struct uc_tlb_entry {
     uc_prot perms;
 };
 
+/*
+ Variables to control which state should be stored in the context.
+ Defaults to UC_CTL_CONTEXT_CPU. The options are used in a bitfield
+ so to enable more then one content the binary or of the required
+ contents can be use.
+ The UC_CTL_CONTEXT_MEMORY stores some pointers to internal allocated
+ memory. Therefor it's not possible to use this context with another
+ unicorn object.
+*/
+
 typedef enum uc_context_content {
     UC_CTL_CONTEXT_CPU = 1,
     UC_CTL_CONTEXT_MEMORY = 2,