Tracking issue for RFC 50: Print statement and string formatting

[wanda-phi]

• RFC PR: Add an RFC for Print statement and string formatting. rfcs#50
• Implementation PR: Implement RFC 50: Print and string formatting. #1190
• Yosys feature implementation PR: Add several new formatting features used by Amaranth to translate Python format strings YosysHQ/yosys#4301
• Yosys version bump PR: Bump Yosys dependency to >=0.40 #1309
• RTLIL backend extension PR: back.rtlil: implement remaining format specifiers. #1280

[wanda-phi]

Most of this is done in #1190; However, yosys doesn't yet support some Format features. The plan is:

• add missing features to yosys, extending its format language
• wait for yosys release
• bump yosys version requirement
• change RTLIL backend to actually emit the newly added format specifiers

The missing features are:

• X type specifier (uppercase hex), which will probably be called H in yosys
• = alignment mode
• arbitrary fill byte (yosys only supports and 0)
• 0, #, and _ options
• as sign
• (usefulness unclear) a mode to print a value as single Unicode character (currently we can only support ASCII via c)