Make Java 8 the minimum requirement for Clojure

Make Java 8 the minimum required version for Clojure.

Patches (all need to be applied):

• 0001-vendor-asm-to-sha-88a0aa8a79df7370cd178281bdf690ac23.patch - bump vendored ASM to 6.2 release

• update-asm-license.patch - updates ASM license in README

• 0002-bump-javac-output-to-1.8-classfiles.patch - switch to compiling jdk8 source/target

• 0003-Compiler-genclass-emit-jdk8-classfiles.patch - change compiler+genclass to emit jdk8 classfiles

• clj-2363-lib-2.patch - update jdk-specific stuff in javadoc (updated jdks), annotation tests (updated jdks), reducers (removed reliance on jsr166 jar), and instant_18 (made non-conditional)

These are all independent and broken up for ease of understanding and independent update as needed.

Screener's Notes

• the ASM vendoring was automated via a bash script (see commit message on 0001-vendor-asm-to-sha-88a0aa8a79df7370cd178281bdf690ac23.patch). I reran and compared.

• the calls to ASM had to change slightly. Assumptions are decisions are in the commit message for 0003-Compiler-genclass-emit-jdk8-classfiles.patch.

• the test changes in clj-2363-lib-2.patch were necessary because the tests were hard-coded against outputs from the annotations API that have expanded over time. These test are not great but are the only coverage of little-used functionality, so meh.