Add support for supplying MIB file directly

[zaccari]

First off, just want to say thank you for creating this library - it has helped me tremendously.

I use a small set of modules on top of a custom MIB, and it is sometimes useful for me to include the MIB files in the go binary, instead of expecting them to be available on the file system (in a container, for example). You can still load via files, if needed, this just adds a little more flexibility.

Example:

gosmi.Init()

gosmi.AppendPath("/you/can/still/load/these/mibs")

for _, moduleName := range moduleNames {
  moduleData, err := GetModuleData(moduleName)
  if err != nil {
    return err
  }

  gosmi.AddModuleFile(moduleName, moduleData)

  _, err = gosmi.LoadModule(moduleName)
  if err != nil {
    return err
  }
}

[sleepinggenius2]

Thank you for your kind words. It is really nice to see the library being put to use within the community.

I like the idea for this PR, but I think the implementation could be even simpler and more generic. The parser already supports reading from an io.Reader, so I think just adding additional functions to get and load a module from an io.Reader instead of a filename would be the more generic option that would still support your use case. That should also support the various bundler/virtual filesystem implementations out there, in addition to raw strings or byte slices. If I'm missing something else about your use case, please let me know.

[zaccari]

I agree - I should have some time over the holidays to simplify things and test. Thank you for reviewing and providing feedback!

[AtakanColak]

I think the new embed package that will come with Go 1.16 should be used for achieving this functionality. Example code;

//go:embed my.mib
var f embed.FS
file, _ := f.Open("my.mib")
module, err := parser.Parse(file)

[sleepinggenius2]

Now that Go 1.16 has landed, can you tell me if v0.4.0 with (io/fs).FS support will meet your needs or do you still have a use case for loading directly from an io.Reader? If so, that should be implementable within your application via the smi.FS interface (=(io/fs).FS in go1.16+).