Issue Description:

Description

Swift.String has the method getLineStart(_:end:contentsEnd:for:) that is corresponding to the same name method in NSString to get the character indexes for its line boundary.

Regarding NSString, this method has a performance advantage compared with lineRange(for:) by passing nil to one or two of the first three parameters, because then, the method doesn't try finding those boundaries. This characteristic is mentioned also in the reference. However, in the Swift.String version, it accepts only non-optional UnsafeMutablePointer<Self.Index>; so there is no way to get only a single boundary of a line range.

This is actually not a bug but undesirable behavior (so please change the issue type if it's wrong). The same issue exists also in getParagraphStart(_:end:contentsEnd:for:).

Expected Result

The parameters start, end, and contentsEnd accept nil so that the method can skip finding needless indexes.