v0.2.1

Hardening fixes only; no API or ABI change (PIO_ABI_VERSION stays 3).

• MATPOWER: a crafted gencost NCOST (e.g. 1e20) overflowed the row
  width arithmetic and panicked on every build profile, a denial of
  service on untrusted input through the Rust API and the CLI. The width
  now saturates and the row is rejected as a ShortRow parse error.
  Found by malformed input fuzzing.
• C ABI: error and warning messages were clipped at a raw byte count,
  which could split a multibyte UTF-8 character and hand the caller an
  invalid string. Truncation now lands on a character boundary.
• PowerWorld .pwd: the reader's byte accessors return Option instead
  of indexing, so an out of range offset from a corrupt file rejects the
  record instead of panicking. A corruption sweep test pins the
  invariant; the differential oracle tests pass unchanged.
• powerio.h: a doc comment contained a literal */ that terminated
  the generated block comment, so compiling with -DPIO_GRIDFM against
  the shipped 0.2.0 header failed with unknown type name 'raw'.

What's Changed

• release: 0.2.1 by @samtalki in #117

Full Changelog: v0.2.0...v0.2.1