Support x packet for binary memory read.

[daniel5151]

GDB recently (Dec 2024) gained support for this packet, via bminor/binutils-gdb@e16e638

Much like #82, which implemented x's sibling packet (X - for binary memory writes), this should be a simple, straightforward packet to add support for. It will simply hook into the existing Target-facing read_memory methods, and work as expected.

One other minor thing to keep in mind: we'll also want to add a new fn use_x_packet(&self) -> bool method to Target, in order to give space-constrained users the ability to opt-out of support for this packet, just as #82 added use_x_upcase_packet when it implemented support for X.

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Note that LLDB has already supported this packet for quite a while now. See https://lldb.llvm.org/resources/lldbgdbremote.html#x-binary-memory-read

Fortunately, GDB and LLDB share the exact same syntax and semantics for this packet, so no extra handling will be required to maintain compatibility between the two clients.