FscHelper: add fsc function

[yawaramin]

The 'fsc' helper function is a simple wrapper for the 'fsc.exe' F#
compiler command-line tool. It takes a source file and an F# list of
options, and compiles the source file.

Fixes #478.

[forki]

This is really cool. I'd like to see this in FAKE.

I think it would be better to encapsulate the compiler switches in a record. This would match the other FAKE tasks.

Is it possible to compile more than one file? I think the default should be to send a fileset to the task.

[yawaramin]

I thought about using a record type for the switches, it would probably look something like this in usage:

// (Previously defined:)
type FscTarget = Library | Exe | Winexe

Target "MyFile" (fun _ ->
  fsc
    { FscOptions() with
      In = ["File1.fs"; File2.fs"]
      Target = Library
      Out = "Compiled.dll"
      Debug = true
      References = ["Base.dll"; "Common.dll"]})

This is definitely more type-safe, but arguably clunkier for the user to write, compared to the simple listing of command-line switches that they're probably already used to.

Using the fsc function as it is right now, it's definitely possible to compile multiple files, but I admit the way I've implemented it right now makes it non-obvious (you just include the extra source files in the options list). I could re-implement to take a single parameter as originally planned, e.g.:

fsc ["File1.fs"; "File2.fs"; "-a"; "-o"; "Compiled.dll"; "-g"; "-r"; "Base.dll"; "-r"; "Common.dll"]

So, ultimately your choice: the record style or the string list style?

[forki]

How about a FSC task (in record style) which uses the existing lower case fsc task.

So we give users the chance to fall back to the args list

[yawaramin]

I've implemented the Fsc function in the same style as the Fake.TypeScript.TypeScriptCompiler function. Fsc can thus be used directly inside targets.

[forki]

I'd like to let the Fsc task take a file set as all the other tasks do.
Can you check if it still works for you?

[yawaramin]

The file set works fine, thanks. I've made some more changes which I think are warranted, will do a PR soon.