It allows a variable size, but in practice we always use the default of
8192 bytes. This commit fixes it to that size, which makes things
slightly faster because the size can be hard-wired in generated code.
The commit also:
- Rearranges some buffer capacity checks so they're all in the same form
(`x > BUFSIZE`).
- Removes some buffer capacity assertions and comments about them. With
an 8192 byte buffer, we're not in any danger of overflowing a `usize`.
f2df861c7f