gnu: tectonic: Update to 0.8.1.

gnu: tectonic: Update to 0.8.1.

* gnu/packages/rust-apps.scm (tectonic): Update to 0.8.1.
[inputs]: Add FREETYPE, GRAPHITE2, ICU4C, LIBPNG.
* gnu/packages/crates-io.scm (rust-pinot-0.1)[origin]<patches>: Add patch
necessary for Tectonic.
(rust-tectonic-docmodel-0.1): Update to 0.1.2.
(rust-tectonic-engine-spx2html-0.1): New variable.
(rust-tectonic-engine-xetex-0.2): New variable.
(rust-tectonic-engine-xetex-0.1): Remove variable.
(rust-tectonic-io-base-0.4): New variable.
(rust-tectonic-io-base-0.3): Inherit from above.
(rust-tectonic-xdv-0.2): New variable.
(rust-tectonic-xdv-0.1): Remove variable.
* gnu/packages/patches/rust-pinot-0.1-implement-math-table.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register new file.