In #5118 we remove the double-underscore to make it a normally-named public
function. However, this is not an interesting function outside of the library
and it takes up a name for something that could be more useful.

Remove the single-underscore version as we have not done any releases with it.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

We don't use double-underscores in the public API.

Move to the `git_error` name in the internal API for error-related
functions.

We use the term "invalid" to refer to bad or malformed data, eg
`GIT_REF_INVALID` and `GIT_EINVALIDSPEC`.  Since we're changing the
names of the `git_object_t`s in this release, update it to be
`GIT_OBJECT_INVALID` instead of `BAD`.

Use the new object_type enumeration names within the codebase.

When failing to parse a raw object fromits data, we free the
partially parsed object but then fail to propagate the error to the
caller. This may lead callers to operate on objects with invalid memory,
which will sooner or later cause the program to segfault.

Fix the issue by passing up the error code returned by `parse_raw`.

Now that we have implement functions to parse all git objects from raw
data, we can implement a generic function `git_object__from_raw` to
create a structure of type `git_object`. This allows us to parse and
interpret objects from raw data without having to touch the ODB at all,
which is especially useful for object verification prior to accepting
them into the repository.

Introduce an internal API to get the object type based on a
length-specified (not null terminated) string representation.  This can
be used to compare the (space terminated) object type name in a loose
object.

Reimplement `git_object_string2type` based on this API.