backed = ffi.strbuf(GCstr *ptr)

This extension will return lua string backed by buffer ptr.
All necessary fields will be filled. Example:

buf = ffi.new [[
	struct {
		uint32_t internal[3];
		uint32_t len;
		char buf[128];
	}"
]]
ffi.copy(buf.buf, "hello")
buf.len = 5
print(buf)

len and buf can be changed at runtime and changes will be reflected
in string contents.

The string will _not_ be interned - which means comparisons
and table indexing is undefined. Use backed:sub(1) to get
proper string copy.

Signed-off-by: katlogic <git@git.git>