[WIP] New $print cell for representing print statements

[whitequark]

This PR introduces a new cell, $print, which can losslessly represent the behavior of Verilog formatting tasks like $display in always blocks, both clocked and continuous. It adds support for parsing Verilog tasks $display (as well as b, o, h suffix variants) and $write (as well as b, o, h suffix variants) such that they are translated to $print cells, and emitting $print cells as appropriate always blocks containing a $write task.

As a side effect, this PR greatly improves conformance with the Verilog LRM for the $display and $write family tasks used in initial blocks, as well as the $sformatf function. Unfortunately, the latter improvement is likely a breaking change.

Internally, this PR adds an abstract representation of format strings with substitutions that supports RTLIL and Veriog (de)serialization. It specifically does not use Verilog format strings anywhere but on the boundary. This is necessary because IEEE 1800-2017 does not precisely define the semantics of almost every aspect of its formatting facility, and real-world implementations wildly disagree with each other on even the grammar they accept.

The format string representation added by this PR is a new one I designed specifically for this task. It heavily borrows from Python f-strings (in particular, the method of escaping the substitutions is exactly identical) and is designed for simplicity of programmatic use and human readability, in that order.

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Remaining work:

• Add tests (diff against iverilog) for the Fmt::render facility used by Yosys to print messages when it encounters initial $display statements and for $sformatf.
• Add tests (roundtrip identity check) for the Verilog/RTLIL translation facilities used to support the $print cell.
• Document the $print cell and its format string mini-language.
• Implement $print cell handling in CXXRTL.
• Add back %m support that existed in the old $display implementation (as an alias of %l) but was removed because it is unclear whether this implementation is correct.
• Add $time and %t support to format strings.

[whitequark]

Likely superseded by #3721. (I'll keep this open for now.)

[whitequark]

Obsoleted by #3721.