[FS-1142] Extended numeric literals

[Tangent-90]

Improve underscores in numeric literals and allow hex, octal, binary custom numeric literals

Click “Files changed” → “⋯” → “View file” for the rendered RFC.

See also:

• Discussion
• Related suggestions:
  • Underscores in numeric literals after prefix and before suffix
  • Extend custom numeric types to support floating point literals
  • and Hexadecimal, octal and binary custom numeric literals
• Implementation