util: Add a string buffer implementation

util: Add a string buffer implementation

Based on Vladislav Egorovs work on the preprocessor, but split
out to a util functionality that should be universal. Setup, teardown,
memory handling and general layout is modeled around the hash_table
and the set, to make it familiar for everyone.

A notable change is that this implementation is always null terminated.
The rationale is that it will be less error-prone, as one might
access the buffer directly, thereby reading a non-terminated string.
Also, vsnprintf and friends prints the null-terminator.

Signed-off-by: Thomas Helland <thomashelland90@gmail.com>
Tested-by: Dieter Nützel <Dieter at nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
V2: Address review feedback from Timothy and Grazvydas
   - Fix MINGW preprocessor check
   - Changed len from uint to int
   - Make string argument const in append function
   - Move to header and inline append function
   - Add crimp_to_fit function for resizing buffer

V3: Move include of ralloc to string_buffer.h

V4: Use u_string.h for a cross-platform working vsnprintf

V5: Remember to cast to char * in crimp function

V6: Address review feedback from Nicolai
   - Handle !str->buf in buffer_create
   - Ensure va_end is always called in buffer_append_all
   - Add overflow check in buffer_append_len
   - Do not expose buffer_space_left, just remove it
   - Clarify why a loop is used in vprintf, change to for-loop
   - Add a va_copy to buffer_vprintf to fix failure to append arguments
     when having to resize the buffer for vsnprintf.

V7: Address more review feedback from Nicolai
   - Add missing va_end corresponding to va_copy
   - Error check failure to allocate in crimp_to_fit