Use write_all for fixed-size payloads in R1CS writer

[MozirDmitriy]

This change replaces two calls to write(...) with write_all(...) when writing fixed-size payloads:

• CustomGatesUsedSection: parameters (field_size bytes each)
• CustomGatesAppliedSection: signal indices (8 bytes each)

The paired reader uses read_exact for these fields, so write(...) risks short writes and corrupting the bytestream. Using write_all(...) guarantees the full buffer is written, ensuring consistency with the reader and with the rest of the module, which already uses write_all for fixed-size data.