new features:
- classfile auto register
- gop mod download
- gop mod tidy
- gop mod init
- gop/env.GOPROOT
- set goproxy when building Go+ in China
- cmd/install.go: getBuildVer automatically
changes:
- mv gop.Version => gop/env.Version
- mv gop.build.Build => gop/env.BuildInfo
- lookup both gop.mod and go.mod
- parser: add TestErrLambda
- optimize cmd/install.go, support uninstall