Make "dead".S work to generate a signed number

**Type of issue**: Feature Request

**Is your feature request related to a problem? Please describe.**
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**Describe the solution you'd like**
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There is "hdead".U. You can't do "hdead".S . And it's hard to do "hdead".U.asSInt because you can't do the latter outside of a Module, you are forced to do 57005.S

**Describe alternatives you've considered**
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Is BigInt("dead", 16).S equivalent/an acceptable alternative.

**Additional context**
Include detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. Stack Overflow, gitter, [Scastie](https://scastie.scala-lang.org/KtzZQ3nFTea9KoNh0tRqtg).

**What is the use case for implementing this feature?**

Annoying to have to write numbers in decimal when hex is more natural, making .S similar to .U