A tree entry previously pointed directly into the object id within the
tree object itself; this is useful to avoid any unnecessary memory copy
(and an unnecessary use of 40 bytes per tree entry) but difficult if we
change the underlying `git_oid` object to not simply be a raw object id
but have additional structure.

This commit moves the `git_oid` directly into the tree entry; this
simplifies the tree entry creation from user data.  We now copy the
`git_oid` into place when parsing.

Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.

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

Use the new object_type enumeration names within the codebase.

The `parse_mode` option uses an open-coded octal number parser. The
parser is quite naive in that it simply parses until hitting a character
that is not in the accepted range of '0' - '7', completely ignoring the
fact that we can at most accept a 16 bit unsigned integer as filemode.
If the filemode is bigger than UINT16_MAX, it will thus overflow and
provide an invalid filemode for the object entry.

Fix the issue by using `git__strntol32` instead and doing a bounds
check. As this function already handles overflows, it neatly solves the
problem.

Note that previously, `parse_mode` was also skipping the character
immediately after the filemode. In proper trees, this should be a simple
space, but in fact the parser accepted any character and simply skipped
over it. As a consequence of using `git__strntol32`, we now need to an
explicit check for a trailing whitespace after having parsed the
filemode. Because of the newly introduced error message, the test
object::tree::parse::mode_doesnt_cause_oob_read needs adjustment to its
error message check, which in fact is a good thing as it demonstrates
that we now fail looking for the whitespace immediately following the
filemode.

Add a test that shows that we will fail to parse such invalid filemodes
now.

When parsing a tree entry's mode, we will eagerly parse until we hit a
character that is not in the accepted set of octal digits '0' - '7'. If
the provided buffer is not a NUL terminated one, we may thus read
out-of-bounds.

Fix the issue by passing the buffer length to `parse_mode` and paying
attention to it. Note that this is not a vulnerability in our usual code
paths, as all object data read from the ODB is NUL terminated.

We currently don't have any tests that directly exercise the tree
parser. This is due to the fact that the parsers for raw object data has
only been recently introduce with commit ca4db5f4 (object: implement
function to parse raw data, 2017-10-13), and previous to that the setup
simply was too cumbersome as it always required going through the ODB.

Now that we have the infrastructure, add a suite of tests that directly
exercise the tree parser and various edge cases.