cstring: avoid excessive growth just to 0-terminate

cstring: avoid excessive growth just to 0-terminate

Based on following what happens in CString::new("string literal"):

1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
   to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.

If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.

Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.