fix: key the persistent code cache on the language level

[marevol]

Stacked on #56.

A script compiled under --language=es6 was served back to an --language=es5 run, which then executed syntax es5 cannot parse:

// over the 1000 character store threshold
var f = (a) => a * 3;
print("arrow result: " + f(1));

command	output
sai --language=es6 --persistent-code-cache big.js	arrow result: 3
sai --language=es5 --persistent-code-cache big.js	arrow result: 3
sai --language=es5 big.js	SyntaxError: Expected ; but found =>

The store directory is named after a digest of the sai jar, with _opt appended for optimistic types. The language level appeared nowhere in it, and Context.compile consults the store before the source is parsed, so the entry was found and the parser never ran.

The same hole exists for -scripting and --no-syntax-extensions and predates this series, but --language is the axis the series turns on: es5 behaviour being unchanged is the whole guarantee, and this broke it. So _es6 now goes into the directory name beside _opt.

getCodeCachePath in the test grew the same distinction, or it could have handed an es6 directory to a test asking for the es5 one.

Verification

./gradlew build testOptimistic testPessimistic

suite	before	after
test	665, 0 fail	666, 0 fail
testOptimistic	1717, 0 fail	1718, 0 fail
testPessimistic	1717, 0 fail	1718, 0 fail