Object parse fixes

Unfortunately, neither the `memmem` nor the `strnstr` functions are part
of any C standard but are merely extensions of C that are implemented by
e.g. glibc. Thus, there is no standardized way to search for a string in
a block of memory with a limited size, and using `strstr` is to be
considered unsafe in case where the buffer has not been sanitized. In
fact, there are some uses of `strstr` in exactly that unsafe way in our
codebase.

Provide a new function `git__memmem` that implements the `memmem`
semantics. That is in a given haystack of `n` bytes, search for the
occurrence of a byte sequence of `m` bytes and return a pointer to the
first occurrence. The implementation chosen is the "Not So Naive"
algorithm from [1]. It was chosen as the implementation is comparably
simple while still being reasonably efficient in most cases.
Preprocessing happens in constant time and space, searching has a time
complexity of O(n*m) with a slightly sub-linear average case.

[1]: http://www-igm.univ-mlv.fr/~lecroq/string/

The function `git__strtol32` can easily be misused when untrusted data
is passed to it that may not have been sanitized with trailing `NUL`
bytes. As all usages of this function have now been removed, we can
remove this function altogether to avoid future misuse of it.

The function `git__strtol64` does not take a maximum buffer length as
parameter. This has led to some unsafe usages of this function, and as
such we may consider it as being unsafe to use. As we have now
eradicated all usages of this function, let's remove it completely to
avoid future misuse.

Our "util.h" header is a grabbag of various different functions, where
many don't have a clear group they belong to. Our set of allocator
functions though can be clearly singled out as a single group of
functions that always belongs together. Furthermore, we will need to
implement additional functions relating to our allocators subsystem when
moving to pluggable allocators. Thus, we should just move these
functions into their own "alloc" module.

Right now, the standard allocator is being declared as part of the
"util.h" header as a set of inline functions. As with the crtdbg
allocator functions, these inline functions make it hard to convert to
function pointers for our allocators.

Create a new "stdalloc" module containing our standard allocations
functions to split these out. Convert the existing allocators to macros
which make use of the stdalloc functions.

Currently, the `git__free` function is being defined in a single place,
only, disregarding whether we use our standard allocators or the crtdbg
allocators. This makes it a bit harder to convert our code base to use
pluggable allocators, and furthermore makes the border between our two
allocators a bit more blurry.

Implement a separate `git__crtdbg__free` function for the crtdbg
allocator in order to completely separate both allocator
implementations.

The diff driver truncates the hunk header text to 80 bytes, which can truncate
4-byte Unicode characters and introduce garbage characters in the diff
output. This change sanitizes the hunk header before it is displayed.

This mirrors the test in git: https://github.com/git/git/blob/master/t/t4025-hunk-header.sh

Closes https://github.com/libgit2/rugged/issues/716

While "util.h" declares the macro `git__tolower`, which simply resorts
to tolower(3P) on Unix-like systems, the <ctype.h> header is only being
included in "util.c". Thus, anybody who has included "util.h" without
having <ctype.h> included will fail to compile as soon as the macro is
in use.

Furthermore, we can clean up additional includes in "util.c" and simply
replace them with an include for "common.h".

use consistent names for the #include / #define header guard pattern.