Add String.stripMargin

[floitschG]

Similar to other languages, although we probably want to just remove all indentation when no margin character is given.

var x = """
  foo() {
    xyz;
  }""".stripMargin();
print(x):

yields:

foo() {
  xyz;
}

With margin character:

var x = """
  |foo() {
  |  xyz;
  |}""".stripMargin("|");
print(x):

or:

var x = """foo() {
          |  xyz;
          |}""".stripMargin("|");
print(x):

Both of these yield the same result.

[lrhn]

Not sure this fits into the core libraries. There are many things you can do to format lines of text, and this one is probably a little too specific (and primarily prompted by our less than than adequate multiline string literals - would be better to just change the multiline string literal to remove shared margins to begin with).