Merge branch 'optimise-string-concat' into 'master'

Optimise concat for String

See merge request !255
2 jobs for master in 3 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #55080
test-nightly

00:01:59

failed #55081
allowed to fail
test-stable

00:01:51

 
Name Stage Failure
failed
test-stable Test
Compiling System.TTS
Compiling System.Time.GenJSON
Compiling System.Signal
Compiling System._Signal
Compiling System.Socket
Makefile:24: recipe for target 'test' failed
make: Leaving directory '/builds/clean-and-itasks/clean-platform/tests/linux64'
make: *** [test] Error 1
ERROR: Job failed: exit code 1