Reflect the double nature of buffers in the names

[akrzemi1]

Buffers offer a dual interface:

1. A range of contiguous buffers: "how many buffers", "append another buffer", "iterate over buffers"
2. A sequence of bytes: "how many bytes across buffers", "any bytes at all?", "first 1024 bytes"

It would be easier for the users if the names reflected which of the two aspects is explointed in a function:

• buffers_begin -- begin of the range of buffers
• buffers_size -- number of buffers
• bytes_size -- number of bytes
• bytes_empty -- no bytes