The six non-deprecated value() overloads each held a full copy of the
same body: the four key-based overloads looked up the key and either
returned the found element converted to the requested type or the
default value (throwing type_error.306 if this is not an object), and
the two json_pointer overloads did the same via
ptr.get_checked_or_null(), throwing type_error.306 unless
is_structured(). Replace the duplicated bodies with two private
helpers, value_member() and value_pointee(), that return a
const basic_json* (null when not found) and do the type check/throw
once each. Every value() overload now just picks between the found
pointer's get<T>() and the default.
Same signatures, template parameters, SFINAE conditions, exception id,
message and this context on every overload.
Overlaps #5689 (routes find() through lookup_key()) and #5705 (adds a
deleted integral-key value() next to these overloads); whichever of
this commit and those PRs lands second will need a small rebase.
#5724 item 4
Signed-off-by: Niels Lohmann <mail@nlohmann.me>