Instead of a `git_bom_t` that a `git_buf` function returns, let's keep
it `git_buf_bom_t`.

The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.

Prefer `off64_t` internally.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

Use the new object_type enumeration names within the codebase.

The callback mechanism makes it awkward to write data from an IO
source; move to `_fromstream()` which lets the caller remain in control,
in the same vein as we prefer iterators over foreach callbacks.

By returning when the count goes to zero rather than below it, setting
`howmany` to 7 in fact writes out the string 6 times.

Correct the termination condition to write out the string the amount of
times we specify.