Use Stringbuilder in Concat function and test performance

[thygesteffensen]

Not needed to fix, but small performance increate using a string builder to append x strings, otherwise you will allokate so0 = s0+s1, then so1 = so0 + s2, so2 = so1 +s3 ect.

can be optimized later (would be cool to have some performance tests/messures) fore optimizing things.

Originally posted by @pksorensen in #75 (comment)