RFR: switch bootstraps

[Brian Goetz]

I've just checked in (to the "switch" branch) bootstraps for linking 
switch callsites, as outlined in "Switch translation, part 1" 
(amber-spec-experts, 12/7/17), for int types (int, short, byte, char), 
their boxes, strings, and enums.

Please review the bootstrap specs, implementation, and test:

http://hg.openjdk.java.net/amber/amber/file/68a36e1e6d98/src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java
http://hg.openjdk.java.net/amber/amber/file/68a36e1e6d98/test/jdk/java/lang/runtime/TestSwitchBootstrap.java

Doesn't yet do float, double, or long -- that's coming.