js: Use gjs_arg_value to get, set and access GIArgument values

This allows to automatically use the proper union value depending on the
type without having to do it manually.

(Philip: changed some formatting, added char32_t for GI_TYPE_TAG_UNICHAR
and changed gjs_arg_value() to gjs_arg_get() if the union member wasn't
needed)
17 jobs for arg-inlines in 5 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Source Check
passed #807588
build_recommended

00:01:05

passed #807589
cppcheck

00:00:30

passed #807590
cpplint

00:01:28

passed #807591
iwyu

00:02:18

 
  Test
passed #807604
build_maximal

00:01:19

passed #807603
build_minimal

00:01:16

passed #807592
privileged
sanitizer_gcc

00:01:02

failed #807593
build_maximal

00:02:55

failed #807594
build_minimal

00:02:36

 
  Manual
manual #807595
allowed to fail manual
coverage
manual #807598
allowed to fail manual
installed_tests
manual #807596
allowed to fail manual
iwyu-full
manual #807597
privileged allowed to fail manual
sanitizer_clang
manual #807599
allowed to fail manual
valgrind
manual #807602
allowed to fail manual
zeal_11
manual #807600
allowed to fail manual
zeal_2
manual #807601
allowed to fail manual
zeal_4