This commit achieves three roughly equally important goals:
1. To bring the rendering code in kernel/fmt.cc and in cxxrtl.h as close
together as possible, with an ideal of only having the bigint library
as the difference between the render functions.
2. To make the treatment of `$time` and `$realtime` in CXXRTL closer to
the Verilog semantics, at least in the formatting code.
3. To change the code generator so that all of the `$print`-to-`string`
conversion code is contained inside of a closure.
There are two reasons to aim for goal (3):
a. Because output redirection through definition of a global ostream
object is neither convenient nor useful for environments where
the output is consumed by other code rather than being printed on
a terminal.
b. Because it may be desirable to, in some cases, ignore the `$print`
cells that are present in the netlist based on a runtime decision.
This is doubly true for an upcoming `$check` cell implementing
assertions, since failing a `$check` would by default cause a crash.
a33acb7cd9