util.jid: Add missing test cases (98% mutant score, single remaining mutant is a string.sub equivalent)

.luacheckrc: Add test_mutants.sh.lua to the ignore list

tools: Add initial mutation testing script

util.dbuffer: Add a bunch of missing test cases Found via mutation testing.

util.dbuffer: Add efficient shortcuts for discard() in certain cases If the buffer is already empty, nothing to do. If we're throwing away the whole buffer, we can just empty it and avoid read_chunk() (which in turn may collapse()). These shortcuts are much more efficient.

util.dbuffer: Remove redundant code (read_chunk() cannot fail at this point)

Merge 0.12->trunk

util.jsonschema: Use same integer/float logic on Lua 5.2 and 5.3 Fixes test case type.json:0:1 covering treatment of 1.0 as an integer according to the JSON definition

util.jsonschema: Fix Lua 5.2 integer compat math.type() is unavailable before Lua 5.3 so this should use the compat function added at the top

util.jsonschema: Ignore test case for JavaScript specific detail Also touching on how arrays are indistinguishable from tables in Lua