This patch started by changing target_read_alloc_1 to return a
byte_vector, to avoid manual memory management (in target_read_alloc_1
and in the callers). To communicate failures to the callers, it
actually returns a gdb::optional<gdb::byte_vector>.
Adjusting target_read_stralloc was a bit more tricky, since it wants to
return a buffer of char, and not gdb_byte. Since you can't just cast a
gdb::byte_vector into a gdb::def_vector<char>, I made
target_read_alloc_1 templated, so both versions (that return vectors of
gdb_byte and char) are generated. Since target_read_stralloc now
returns a gdb::char_vector instead of a gdb::unique_xmalloc_ptr<char>, a
few callers need to be adjusted.