Add Object API which, among other things, can be used for peeking

EXT types and sizes and sizes were swapped when writing and reading
them; fixed and updated tests

Clean up test code formatting; no line exceeds 80 chars now

Remove undefined behavior from test buffer code

Remove custom 'dboolean' bool type from test code

Remove custom 'byte' uint8_t type from test code

Replace Doom-specific I_Error function with errorf_and_exit function
that does the same thing

Move error_and_exit to utils.c so the buffer can make use of it

Add function names and line numbers to test errors