It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.
For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
* introduce std.json.WriteStream API for writing json
data to a stream
* add WIP tools/merge_anal_dumps.zig for merging multiple semantic
analysis dumps into one. See #3028
* add std.json.Array, improves generated docs
* add test for `std.process.argsAlloc`, improves test coverage and
generated docs