make String.f32() and String.f64() partial

[mfelsche]

and fail if the string at the given offset contains an invalid F32/F64
which includes the empty string and trailing characters.

previously they were returning 0 in their respective return-type. So it was impossible to find out if parsing failed.

fixes #3026

[mfelsche]

Hey, everything succeeded but the docker build check failed because it cant find some makefile. That is unrelated to this PR and needs some separate investigation.

[SeanTAllen]

@mfelsche its because your PR isnt against the latest master.

[SeanTAllen]

Please do not merge this prior to the 0.27.0 release. There's no release notes with "before and after" code for the breaking change and I don't want to hold up the release that I am planning on doing within 4-5 hours.

[mfelsche]

@SeanTAllen here are my release notes:

This change changes the methods builtin.String.f32(offset: ISize) and builtin.String.f64(offset: ISize) to be partial and fail on input that cannot be parsed as a floating point number. This is the current behavior already for parsing integers from Strings.

Previously the methods for parsing floats would just return 0.0 on invalid input, overflow or when the given offset was beyond the bounds of the String it was invoked on. As 0.0 is a valid float, there was no real way to determine if the parsing failed or if 0.0 is a legit parsed value. These examples try to show the behavior of Pony prior to 0.27.0:

// true with Pony 0.26.0 and older
"".f32() == F32(0)
"ABC".f64() == F64(0)
"0.123".f64(10) == F64(0) 

The following example shows the new behavior with Pony 0.27.0 and newer:

// these expressions will error with Pony 0.27.0 and newer
try "".f32()? end
try "ABC".f64()? end
try "0.123".f64(10)? end