Make Solver object safe

[jjbayer]

Replace associated constant for solver names with a method to make Solver object safe.

A trait is object safe if it has the following qualities (defined in RFC 255):

• [...]
• It must not have any associated constants.

https://doc.rust-lang.org/beta/reference/items/traits.html#object-safety

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 7.95455% with 81 lines in your changes are missing coverage. Please review.

Project coverage is 92.11%. Comparing base (73c52ce) to head (d526949).

Files	Patch %	Lines
crates/argmin/src/core/test_utils.rs	0.00%	3 Missing ⚠️
crates/argmin/src/solver/brent/brentopt.rs	0.00%	3 Missing ⚠️
crates/argmin/src/solver/brent/brentroot.rs	0.00%	3 Missing ⚠️
crates/argmin/src/solver/conjugategradient/cg.rs	0.00%	3 Missing ⚠️
...rgmin/src/solver/conjugategradient/nonlinear_cg.rs	0.00%	3 Missing ⚠️
...n/src/solver/gaussnewton/gaussnewton_linesearch.rs	0.00%	3 Missing ⚠️
...rgmin/src/solver/gaussnewton/gaussnewton_method.rs	0.00%	3 Missing ⚠️
...rates/argmin/src/solver/goldensectionsearch/mod.rs	0.00%	3 Missing ⚠️
crates/argmin/src/solver/landweber/mod.rs	0.00%	3 Missing ⚠️
...rates/argmin/src/solver/linesearch/backtracking.rs	0.00%	3 Missing ⚠️
... and 18 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
- Coverage   92.42%   92.11%   -0.31%     
==========================================
  Files         178      178              
  Lines       24314    24398      +84     
==========================================
+ Hits        22471    22475       +4     
- Misses       1843     1923      +80     

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[jjbayer]

Requires #488 to pass CI.

[stefan-k]

Thanks a lot!