gnu: librsvg-next: Substitute more crates.
* gnu/packages/gnome.scm (librsvg-next-source): Add substitutions for
rust-approx-0.3, rust-arrayvec-0.4, rust-bstr-0.2, rust-byteorder-1.3,
rust-cast-0.2, rust-cloudabi-0.0, rust-criterion-0.2,
rust-criterion-plot-0.3, rust-crossbeam-dequeue-0.7,
rust-crossbeam-epoch-0.7, rust-crossbeam-queue-0.1,
rust-crossbeam-utils-0.6, rust-csv-1.1, rust-csv-core-0.1,
rust-either-1.5, rust-encoding-0.2, rust-fuchsia-cprng-0.1,
rust-futf-0.1, rust-generic-array-0.12, rust-itertools-0.8,
rust-language-tags-0.2, rust-mac-0.1, rust-matches-0.1,
rust-new-debug-unreachable-1.0, rust-num-complex-0.2, rust-phf-0.7,
rust-phf-codegen-0.7, rust-phf-generator-0.7, rust-phf-shared-0.7,
rust-pkg-config-0.3, rust-recomputed-hash-0.1, rust-rand-0.6,
rust-regex-automata-0.1, rust-rustc-version-0.2, rust-semver-09,
rust-semver-parser-0.7, rust-siphasher-0.2, rust-smallvec-0.6,
rust-string-cache-0.7, rust-string-cache-shared-0.3,
rust-thread-local-0.3, rust-tinytemplate-1.0, rust-typenum-1.10,
rust-unicode-bidi-0.3, rust-unicode-normalization-0.1, rust-utf-8-0.7,
rust-winapi-i686-pc-windows-gnu-0.4,
rust-winapi-x86-64-pc-windows-gnu-0.4.
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