fread: change return types of all field parsers

[st-pasha]

Previously all field parsers returned _Bool status, with true indicating a successful parse, and false indicating a failure.

This PR makes it so that all field parsers return an int, with 0 indicating success and 1 indicating failure. This is equivalent to applying logical not to all return values. The reason for such change is that it will allow us to have more than one type of failure in the future (i.e. 1 indicating regular failure, 2 indicating recoverable failure, etc.) These extend return types will be used in PR #2200

Additionally, this PR modifies the type of the target parameter: each field parser will have the target variable suitable for that particular parser. This improves code readability and serves as additional documentation as to the purpose of each parser.

In some places where trash variable were declared as char[8] those were replaced with double or int64_t or similar -- this is to ensure proper alignment of the variable (char[8] may be byte-aligned, whereas double is aligned to 8-byte boundary).

[codecov-io]

Codecov Report

Merging #2234 into master will increase coverage by <.01%.
The diff coverage is 98.14%.

@@            Coverage Diff             @@
##           master    #2234      +/-   ##
==========================================
+ Coverage   90.71%   90.71%   +<.01%     
==========================================
  Files          59       59              
  Lines       11507    11508       +1     
==========================================
+ Hits        10438    10439       +1     
  Misses       1069     1069

Impacted Files	Coverage Δ
src/fread.c	93.02% <98.14%> (ø)	⬆️

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