Build fails with --no-default-features

[chrysn]

Both in the released and the latest main version, building without the default combinators feature fails:

$ cargo check --no-default-features
[...]
error[E0432]: unresolved imports `crate::decoder::is_any`, `crate::decoder::CBORDecoder`
  --> tps_minicbor/src/encode.rs:32:22
   |
32 | use crate::decoder::{is_any, CBORDecoder, SequenceBuffer};
   |                      ^^^^^^  ^^^^^^^^^^^ no `CBORDecoder` in `decoder`
   |                      |
   |                      no `is_any` in `decoder`

It's not a big practical issue (I depended on default-features=false more out of habit, and need combinators anyway), but it's weird given that apparently some effort was spent on keeping them optional, but then it's never tested.