VL_STREAML_FAST_ funcs work with log2(rd)

When calling VL_STREAML_FAST_ funcs, pass in log2(rd) instead of rd.
Added an AstStreamL visitor in V3EmitC.cpp (and simplify emitC in
V3AstNodes.h).
replace sizeof calls in VL_STREAML_FAST funcs

Use VL_WORDSIZE/VL_QUADSIZE instead of calls to sizeof.