fix: make `IpcMessage` members non-public #182

Job	Run time
aarch64-apple-darwin	44s
aarch64-apple-darwin force-inprocess	1m 1s
aarch64-apple-darwin async	45s
x86_64-apple-darwin	1m 50s
x86_64-apple-darwin force-inprocess	1m 29s
x86_64-apple-darwin async	1m 24s
x86_64-unknown-linux-gnu	50s
x86_64-unknown-linux-gnu force-inprocess	48s
x86_64-unknown-linux-gnu async	1m 2s
x86_64-pc-windows-msvc	1m 51s
x86_64-pc-windows-msvc force-inprocess	2m 35s
x86_64-pc-windows-msvc async	2m 53s
i686-pc-windows-msvc	4m 7s
i686-pc-windows-msvc force-inprocess	3m 23s
i686-pc-windows-msvc async	3m 50s
x86_64-unknown-linux-gnu memfd	54s
Result	1s
	29m 27s

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