`async` keyword not highlighted

[canndrew]

Self-explanatory. In "pub async fn" only "pub" and "fn" are highlighted as keywords.

• rust.vim version: master (7e1e2d2)
• :RustInfo output:

 RustInfo
rust.vim Global Variables:

let g:ftplugin_rust_source_path = v:null
rust.vim Global Variables:
let g:ftplugin_rust_source_path = v:null
let g:loaded_syntastic_rust_cargo_checker = 1
let g:loaded_syntastic_rust_filetype = v:null
let g:loaded_syntastic_rust_rustc_checker = 1
let g:rust_bang_comment_leader = v:null
let g:rust_cargo_avoid_whole_workspace = v:null
let g:rust_clip_command = v:null
let g:rust_conceal = v:null
let g:rust_conceal_mod_path = v:null
let g:rust_conceal_pub = v:null
let g:rust_fold = v:null
let g:rust_last_args = v:null
let b:rust_last_args = []
let g:rust_last_rustc_args = v:null
let b:rust_last_rustc_args = []
let g:rust_original_delimitMate_excluded_regions = v:null
let g:rust_playpen_url = v:null
let g:rust_prev_delimitMate_quotes = v:null
let g:rust_recent_nearest_cargo_tol = v:null
let g:rust_recent_root_cargo_toml = v:null
let b:rust_recent_root_cargo_toml = 'Cargo.toml'
let g:rust_recommended_style = v:null
let g:rust_set_conceallevel = v:null
let g:rust_set_conceallevel=1 = v:null
let g:rust_set_foldmethod = v:null
let g:rust_set_foldmethod=1 = v:null
let g:rust_shortener_url = v:null
let g:rustc_makeprg_no_percent = v:null
let g:rustc_path = v:null
let g:rustfmt_autosave = 0
let g:rustfmt_autosave_because_of_config = v:null
let g:rustfmt_autosave_if_config_present = v:null
let g:rustfmt_command = 'rustfmt'
let g:rustfmt_emit_files = 0
let g:rustfmt_fail_silently = 0
let g:rustfmt_options = ''
let g:syntastic_extra_filetypes = ['rust']
let g:syntastic_rust_cargo_fname = ''
error: toolchain 'nightly-x86_64-unknown-linux-gnu' does not have the binary `rustfmt`
rustc 1.32.0-nightly (00e03ee57 2018-11-22)
cargo 1.32.0-nightly (b3d0b2e54 2018-11-15)

NVIM v0.3.1
Build type: Release
LuaJIT 2.1.0-beta3
Compilation: /nix/store/7qv7drgc2jadd5r0xi3mfvvf570v2r7s-gcc-wrapper-7.3.0/bin/gcc -Wconversion -O2 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wimplic
it-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -Wno-array-bounds -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/build/source/build/c
onfig -I/build/source/src -I/nix/store/cr14xxbjmj0jry49c24i3p4ldmdvigl4-libuv-1.23.0/include -I/nix/store/8kfvc5kyj87vb2vas5c2572zxmjnnjzh-msgpack-3.0.1/include -I/nix/store/zgjpvi14jy5i0jpvj6rfjbxc44d9cc8n-unibi
lium-2.0.0/include -I/nix/store/kgks6hdjgshxklzkq4a2dpgjv8jiyl13-libtermkey-0.20/include -I/nix/store/vpih96fdjxh0kzb1vcfrp9ll36qkzjvc-neovim-libvterm-2017-11-05/include -I/nix/store/4mh1qp099sy0dk7s49lrjb1x2wynl
xzl-jemalloc-5.0.1/include -I/nix/store/m0vaipbfiqwyckrlbdrhdsa3agzags2i-glibc-2.27-dev/include -I/build/source/build/src/nvim/auto -I/build/source/build/include
Compiled by nixbld

Features: +acl +iconv +jemalloc +tui
See ":help feature-compile"

   system vimrc file: "$VIM/sysinit.vim"
  fall-back for $VIM: "/nix/store/gzs2y6w1kjk7s88va2ddxxlg52sharyv-neovim-unwrapped-0.3.1/share/nvim"

Run :checkhealth for more info

[da-x]

Fixed just now in merged #284.