closes #3913

Internally used in nafill function coerceFill has been heavily modified, and renamed to coerceClass. Main purpose of the function is to handle coercion between types/classes with care on int64.
Comparing to previous coerceFill version, new coerceClass:

• handle coerce of vector, not only scalar
• handle coercion from any class to any class
• int64 coercion is taken care for integer and double only
• coercion between int / double / int64 is handled in the function, otherwise R's coerceVector is used - as a result, consider char to int64 (coerceClass("1", list(integer(0), bit64::as.integer64(0)))). This is going to be addressed in this PR, placeholders are in place. Also there might be some missing warnings like (NA introduced by coercion, Int overflow), which are currently not checked/issued in our int / double / int64 handling.

utils.c tests moved into own test script

motivation:

• extend nafill for types other than integer, numeric and int64
• to be re-used as efficient way to handle conversions like

data.table(a=1:2, b=bit64::as.integer64(1:2))[2L, c("a","b") := NA][]
#       a                   b
#   <int>               <i64>
#1:     1                   1
#2:    NA 9218868437227407266
data.table(a=1:2, b=bit64::as.integer64(1:2))[2L, c("a","b") := coerceClass(NA, .SD)][]
#   <int> <i64>
#1:     1     1
#2:    NA  <NA>

in assign, rbindlist and potentially other places where we need to take care of int64 NAs handling or other classes like #3654

As of current moment I don't understand coverage issue, it reports few closing brackets } that are not being covered. Maybe a matter of waiting for update.