replace ioutil with os,io

[meowsbits]

make lint complains that as of Go 1.16, ioutil is deprecated and is to be replace by os and io.

These errors look like this:

make lint                                                                                                                                                                                      
env GO111MODULE=on go run build/ci.go lint                                                                                                                                                     
build/cache/golangci-lint-1.46.2-linux-amd64.tar.gz is up-to-date                                                                                                                              
>>> build/cache/golangci-lint-1.46.2-linux-amd64/golangci-lint run --timeout 3m0s --config .golangci.yml ./...
tests/params.go:23:2: SA1019: package io/ioutil is deprecated: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details. (staticcheck)                                                                                                               
        "io/ioutil"                                                                                                                                                                            
        ^                                                                                                                                                                                      
tests/difficulty_mgen_test.go:23:2: SA1019: package io/ioutil is deprecated: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details. (staticcheck)
        "io/ioutil"                                                                               
        ^                                                                                                                                                                                      
tests/state_mgen_test.go:23:2: SA1019: package io/ioutil is deprecated: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be 
preferred in new code. See the specific function documentation for details. (staticcheck)                                                                                                      
        "io/ioutil"                                                                                                                                                                            
        ^                                                                                                                                                                                      
util.go:47: exit status 1                                                                                                                                                                      
exit status 1                                                                                                                                                                                  
Makefile:108: recipe for target 'lint' failed                                  
make: *** [lint] Error 1              

[ziogaschr]

This is one is useful for upstream as well :)