gnu: rust: Bootstrap (only) Rust 1.19.0 by mrustc.

gnu: rust: Bootstrap (only) Rust 1.19.0 by mrustc.

* gnu/packages/patches/rust-1.19-mrustc.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/rust.scm (rust-1.19)[source]: Add patch
"rust-1.19-mrustc.patch".
[arguments]<#:modules>: New field.
<#:phases>[patch-cargo-tomls]: New phase.
<#:phases>[build]: Modify.
<#:phases>[install]: Modify.
[native-inputs]: Replace rust-bootstrap by mrustc.
(rust-1.23)[native-inputs]: New field.
[arguments]<#:phases>: Delete phase "patch-cargo-tomls".