Improve fread parser for floats

[st-pasha]

This PR allows fread to recognize floating-point literals with arbitrary many digits, and parse them as doubles. This includes numbers such as 10000000000000000000000000000.0 or 2.23498720349871093847109387e-21, but not 100000000000000000000000000 (long integer will still be parsed as string).

The reason for this change is that we have already seen multiple datasets where floating-point values are stored with more than the "canonical" 17 significant digits. To an unsuspecting user, such number looks completely innocuous: eg. 2.49102793750273308, and it comes at a complete surprise that fread would parse it as a string... It is also relatively easy to produce these extra digits of precision "by accident" asking for too many digits in printf(), or printing with %f format, etc.

This PR also fixes issue #2625 by being more careful about checking whether the literal is valid.

Closes #2625

[codecov-io]

Codecov Report

Merging #2629 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2629      +/-   ##
==========================================
+ Coverage   93.03%   93.04%   +<.01%     
==========================================
  Files          61       61              
  Lines       12116    12131      +15     
==========================================
+ Hits        11272    11287      +15     
  Misses        844      844

Impacted Files	Coverage Δ
src/fread.c	97.14% <100%> (+0.03%)	⬆️

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[MichaelChirico]

Just noticed this in examples(fread)

DT = fread("A\n1.010203040506070809010203040506\n")  # too precise for double, so read as character
# TODO: add numerals=c("allow.loss", "warn.loss", "no.loss") from base::read.table
typeof(DT$A)=="character"   # TRUE

Seems related

[mattdowle]

Great catch and fix.