gdb/record: Refactor record history

This is the first step in a large refactor in how GDB keeps execution
history.  Rather than using a linked list where multiple entries can
describe a single instruction, the history will now be stored in an
std::deque, each instruction being one entry in the deque.

The choice was initially to use an std::vector, but it would become
unwieldy because it needs all the memory to be consecutive, which is
hard for 200 thousand entries. Deque was picked because it was a nice
midpoint between vector (maximum cache cohesion) and linked list
(maximum ease of finding space to store more).

Each instruction in memory will be now one record_full_instruction
entry, which for this commit just contains a vector of
record_full_entry for the effects of the instruction, and the data that
was stored in the record_full_end entry (that is, the instruction number
and the signal, if any).

This change introduced a minimal performance improvement (what's
important is that it isn't a degradation) and a  reduction in the total
memory footprint of roughly 20% if the entire history is used.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Guinevere Larsen <guinevere@redhat.com>