[fpc-pascal] StringList micro optimization

[Michael Van Canneyt]

On Fri, 8 Nov 2019, AlexeyT via fpc-pascal wrote:

> Procedure TStrings.SetDelimitedText(const AValue: string);  has about 16 
> calls to Length(AValue), so make a variable NLen to speed up it.

Optimized, thanks for the hint.

Also reworked it so AddDelimitedText reuses DelimitedText code, no temp stringlist is created
any more.

Michael.