Add string-to-float to common-dylan

[cgay]

common-dylan has integer-to-string, string-to-integer, and float-to-string, but no string-to-float. Look at copying https://github.com/dylan-hackers/gwydion/blob/master/common/common-dylan/float.dylan over to Open Dylan

[cgay]

This is needed for the json parser.

[hannesm]

a remark from @housel on IRC was: that specific code needs bignums, which are not there in Open Dylan

[waywardmonkeys]

There is also an atof in the dfmc-reader ... and I think there's a copy of that code in the LLVM asm parser as well!

[cgay]

https://arxiv.org/pdf/2101.11408.pdf