Basic support for FFI callbacks (at least stop segfaulting and report a proper error) #12403

Job	Run time
coverage report	5m 44s
bootstrap build	2m 58s
style checks	1m 6s
test (i686-unknown-linux-gnu)	10m 11s
test (aarch64-unknown-linux-gnu)	7m 14s
test (x86_64-unknown-linux-gnu)	12m 40s
test (aarch64-apple-darwin)	9m 13s
test (i686-pc-windows-msvc)	2m 33s
test (aarch64-pc-windows-msvc)	1m 59s
test (armv7-unknown-linux-gnueabihf)	10m 35s
automatic pull from rustc	0s
conclusion	2s
cronjob failure notification	0s
	1h 4m 15s

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