core:encoding/base64 can only produce padded output, so callers that need
the canonical unpadded ("raw") form -- JOSE base64url, Go's
RawStdEncoding/RawURLEncoding -- have to encode and then trim, which
forces an extra allocation and cleanup dance around the returned string.
Add an Encode_Options bit set, mirroring Decode_Options, and honor
{.No_Padding} in `encode`, `encode_into_buf`, `encode_into`, and
`encoded_len`. Existing calls and padded output are unchanged.
Tests cover the RFC 4648 section 9 illustrations and section 10 test
vectors across the standard and URL alphabets, padded and raw, plus Go's
reference corpus. Newline characters are asserted to be rejected per
RFC 4648 section 3.3. Comments in `encode_impl` document the 24-bit group
packing and the padding decision for partial groups.
core:encoding/base64 currently decodes leniently: it infers padding from the
last one or two bytes, accepts missing/extra/interior padding, and does not
check the trailing padding bits required by RFC 4648 Section 3.5.
Go exposes canonical decoding as `Encoding.Strict()`; Odin has no equivalent,
so downstream ports (e.g. age) have to reimplement base64 from scratch.
This adds opt-in strict decoding through a `Decode_Options` bit set.
Existing calls and behavior are unchanged.
Ihor Antonov