string to float not roundtrip for float32

[timotheecour]

now that dragonbox was merged, string to float is roundtrip for float64, but not for float32;
even though dragonbox algorithm can handle it, the one that was ported (dragonbox.cc) only handles float64 (and conversion from float32 to float64 doesn't make it roundtrip)

Example

var a = 1.1'f32
doAssert $a == "1.1", $a # fails

Current Output

assert fails

Expected Output

works

Possible Solution

port or wrap https://github.com/abolz/Drachennest/blob/master/src/schubfach_32.cc, as explained in #18139 (comment), #18008 (comment) as i did via wrapping in timotheecour#732

Additional Information

1.5.1 63db2b1