regcache_cooked_read_ftype can be converted to a function_view, which
allows us to use lambda functions and therefore avoid having to pass an
opaque pointer parameter.
Adjusting the fallouts showed that the "const regcache &" passed to the
readonly_detached_regcache constructor is cast to non-const in
do_cooked_read. I changed the constructor parameter to be non-const.
Finally, I renamed the typedef from regcache_cooked_read_ftype to
register_read_ftype, since there is nothing that forces us to use it
only for regcaches nor cooked registers.