TODO

1. Support random program generation without skeletons (loop and statement),
   code bricks, and initializers.

   For a general framework, we need to abstract CodeSyn as a general
   synthesizer interface providing synExpr(), synStmt(), SynCodeSeg(),
   symBlock(), synLoop(), etc. The skeleton based CodeSyn then becomes
   SklCodeSyn and we should provide RndCodeSyn the synthesizer that generates
   programs from scratch.

   For RndCodeSyn, we have to synthesize code that can trigger diverse opt
   passes in a JIT compiler. For this, refer to CSmith for a general generation,
   YARPGen v.1 for scalar optimizations, and YARPGen v.2 for loop optimizations
   especially various loop idioms leveraging loop sequence, nests, and arrays.