Add rwkv_set_print_errors and rwkv_get_last_error (#68)

* Add rwkv_set_print_errors and rwkv_get_last_error

Fixes #63

This allows retrieving errors from the library without having to
pipe stderr. Also it was annoying that rwkv.cpp assumed control of
the caller process by doing things like calling abort() when it
shouldn't, so I also fixed that.

The basic way this works is:

1. by default, not much is different, except more errors are caught,
   and rwkv.cpp should never abort the process or throw a C++
   exception.

2. the difference comes when you call rwkv_set_print_errors
   (working title):

   1. errors will no longer be printed to stderr automatically
   2. errors will be assigned to a thread-local variable (during
      init/quantization) or a context-local variable (during eval)
   3. the last error can be retrieved using rwkv_get_last_error

I also overhauled the assert macros so more error cases are
handled:

- the file is now closed if rwkv_init_from_file exits early
- the ggml context is freed if rwkv_init_from_file exits early
- if parameters cannot be found an error will be set about it

I also made some optimizations:

- just use fstat instead of opening the file twice
- deduplicated some code / removed edge cases that do not exist
- switched to ggml inplace operations where they exist

test_tiny_rwkv.c seems to run perfectly fine. The Python scripts
also.

The built DLL is perfectly backwards compatible with existing API
consumers like the python library, because it does not remove or
change any functions, only adds some optional ones.

The sad thing is that this will break every PR because the error
handling in this library was terrible and needed to be totally
redone. But I think it is worth it.

* Fix typo

Co-authored-by: Alex <saharNooby@users.noreply.github.com>

* Visual Studio lied and _fileno is incorrect

* Fix trailing comma in assert macros

This was an accident left over from something that didn't pan out,
some compilers do not like when function arguments have a trailing
comma.

* Include header file for fstat

* Remove uses of std::make_unique

* Fix width of format string argument on all platforms

* Use C free for smart pointers

* Revert "Use C free for smart pointers" and try nothrow

* Initialize cgraph to zero

* Fix ggml_cgraph initialization

* Zero-initialize allocations

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Co-authored-by: Alex <saharNooby@users.noreply.github.com>