Fix signature error message

[FabijanC]

Description

In Pythonic cairo-vm, invalid signature was reported like this:

Error at pc=0:124:
Signature (1193313242338130160318297312666865822293235647311047835298033290753491994440, 155691799158656286221600221734062249357606752541106292108852403284952061129),
is invalid, with respect to the public key 3571077580641057962019375980836964323430604474979724507958294224671833227961,
and the message hash 1308364852790706196451346173687342589479412849559424395391024201449978019648.
Cairo traceback (most recent call last):
Unknown location (pc=0:577)
Unknown location (pc=0:541)
Unknown location (pc=0:245)

And in Rust cairo-vm, it's reported like this:

Error at pc=0:124:
Signature 03e4c994748ab4c6a0374e7aa6ae938b6c880a58ff2938ddfdd569bc694351990128cf025da10fcc6742f5c242922fb251c0852c7f68c70e1923fcc74f9454fb,
is invalid, with respect to the public key 2554664058264077495144519976141551974382939217789466639725771495363739387604,
and the message hash 3232999915494270110450164060897339602579794848593724915613880343018894519633.
Cairo traceback (most recent call last):
Unknown location (pc=0:615)
Unknown location (pc=0:600)
Unknown location (pc=0:245)

Notice how the signature is currently two concatenated hexadecimal representations (difficult to separate manually/visually), whereas it used to be two separated decimal representations (consistent with public key and message hash). This PR aims to address this.

I removed the checklist because I don't think it's needed for this small PR.

[codecov]

Codecov Report

Merging #1361 (511b26e) into main (eab16c8) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #1361   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files          93       93           
  Lines       37854    37854           
=======================================
  Hits        36904    36904           
  Misses        950      950           

Files Changed	Coverage Δ
vm/src/vm/runners/builtin_runner/signature.rs	96.73% <0.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more