Improve printing of nested expressions #6176

Job	Run time
Cross compiler for s390x	32m 41s
Test in OpenBSD	9m 6s
testinstall - - ubuntu-latest	5m 43s
teststandard - - ubuntu-latest	20m 30s
testmanuals - - ubuntu-latest	6m 1s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	7m 41s
testpackages testinstall-loadall - - ubuntu-latest	27m 28s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	6m 16s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 29s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 57s
testmockpkg testinstall testworkspace - BOOTSTRAP_MINIMAL=yes - macos-latest	7m 16s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	29m 10s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 49s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 20s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug" - ubuntu-20.04	5m 8s
testmockpkg testinstall - - windows-2019	17m 57s
Send Slack notification on status change	0s
	3h 30m 32s

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