Add more string manipulation functions to the String module [ RFC FS-1033 ]

**Submitted by TheInnerLight on 5/30/2016 12:00:00 AM**
**53 votes on UserVoice prior to migration**

The Core.String module does not provide nearly enough features at present, too often we have to revert to using the the standard .NET string class which both hinders tidy piping and stops us taking advantage of curried args / partial application.
I suggest that at least the following functions be added to the string module:

```
empty : string
isEmpty : string -> bool
isWhitespace : string -> bool
replace : string -> string -> string -> string
startsWith/endsWith : string -> bool
split : seq -> string -> seq
toUpper/toLower(Invariant) : string -> string
trim : string -> string
trimStart/trimEnd : string -> string
```

Obviously all of this can easily be achieved by writing simple wrappers to the methods in the .NET string class but if F# is going to have a String module, it ought to be a fully featured one.

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/14411874)**
[Archived Uservoice Comments](../tree/master/archive/suggestion-14411874-add-many-more-string-manipulation-functions-to-th.md#comments)