This reverts commit 2c0aa257.

so that the "list metadata" command doesn't print out anything
about intrinsics, but other code can see them.

Closes #2771

Conflicts:
	src/snapshots.txt

Tiny documentation fixes in rust.md and src/libcore/task.rs

Add map::clear

Fix metadata serialization of foreign functions. Properly take the value of foreign functions from other crates to fix #1840.

change borrowck error msg: 'declared in outer block' -> 'captured in a closure' (properly this time)

Add clear to the map interface, and implement it in hashmap and smallintmap.