Changes since v0.2.1:

• Add check subcommand
• Add the Web targets: --webjs/--webjs-cps
• Add String16 and related structures in $(SML_LIB)/basis/text16.mlb
• JavaScript: Add JavaScript.Error, JavaScript.isTruthy, JavaScript.isNullOrUndefined, JavaScript.fromString16, JavaScript.Promise, JavaScript.async/JavaScript.await
• Lua: Add Lua.isTruthy, Lua.newTableWith, Lua.newTableWithMetatable, and Lua.call4/.../Lua.call9/Lua.method4/.../Lua.method9