Continuous fuzzing (#944)

Enables automated fuzzing on Fuzzit. Runs fuzz regression tests
every push and PR. Runs full fuzzing every push. Fuzzit emails
if it finds crashes.

Uses the existing fuzz targets:
* translate-module - Fuzz valid WebAssembly modules.
* reader-parse - Fuzz IR text format parsing.

cranelift/ci/fuzzit.sh

@@ -0,0 +1,38 @@
+#!/bin/bash
+set -xe
+
+# Validate arguments
+if [ "$#" -ne 1 ]; then
+    cat << EOF
+Usage: $0 <type>
+
+Types are:
+local-regression - Run corpus and past crashes locally to catch regressions.
+fuzzing - Submit for long run fuzzing on Fuzzit.
+EOF
+    exit 1
+fi
+
+# Configure
+set -xe
+NAME=cranelift
+TYPE=$1
+FUZZIT_VERSION=2.4.46
+
+# Setup
+if [[ ! -f fuzzit || ! `./fuzzit --version` =~ $FUZZIT_VERSION$ ]]; then
+    wget -q -O fuzzit https://github.com/fuzzitdev/fuzzit/releases/download/v$FUZZIT_VERSION/fuzzit_Linux_x86_64
+    chmod a+x fuzzit
+fi
+./fuzzit --version
+
+# Fuzz
+function fuzz {
+    FUZZER=$1
+    TARGET=$2
+    cargo fuzz run $FUZZER -- -runs=0
+    ./fuzzit --version
+    ./fuzzit create job --type $TYPE $NAME/$TARGET ./fuzz/target/x86_64-unknown-linux-gnu/debug/$FUZZER
+}
+fuzz fuzz_translate_module translate-module
+fuzz fuzz_reader_parse_test reader-parse