go vet and lint

* Naming
    * If this is not some OpenPGP standard I follow rule that `DES` should be
      upper case as it is abreviation and `Triple` should be camel-case as it
      is normal word hence `TripleDES`
    * rename `errors2` -> `errorsPGP`
* long lines
    * https://github.com/golang/go/wiki/CodeReviewComments#line-length
    * I bit improved long lines based on my folding
    * reuse type in definition if possible i.e. `a string, b string, c string` -> `a,b,c string`
    * `if long_statetent(); err!=nil {` -> `long_statement;↵ if err!=nil {`
    * spaces around operators (e.g. `a + b` -> `a+b`)
* removing empty lines on start and end of scope
* comments
    * on all exported functions
    * start with function name
* import:
    * order in alphabet
    * separate native, golang.org/x/ and our libs

.gitlab-ci.yml

@@ -22,6 +22,7 @@ test-all:
         - go test -coverprofile cover.out ./...
         - mkdir reports
         - go tool cover -html=cover.out -o reports/cover.html
+        - go run vendor/github.com/golangci/golangci-lint/cmd/golangci-lint/main.go run crypto/... --enable-all -D govet -D interfacer -D gofmt -D prealloc -D gochecknoglobals -D goimports -D gosec -D gocritic -D gochecknoinits
     tags:
         - coverage
     artifacts: