We need to be careful to not grab an extra reference when mutating
data structures because that means we make extra copies. Doing that
every time in `jv_setpath()` is really painful, as that function is used
in `_modify/2`, which is used in `|=` and all modify-assign operators.
`_modify` also grabs additional references, so this is not the only fix
needed for the modify-assign operators to not be accidentally quadratic.
Extend jv_number to use decNumber for storing number literals. Any math
operations on the numbers will truncate them to double precision.
Comparisons when both numbers are literal numbers will compare them
without truncation.
Delay conversion of numbers to doubles until a math operation is performed,
to preserve precision. A literal jv_number will only need conversion to
double once, and will reuse the resultant double on subsequent
conversions.
Outputting literal jv_numbers preserves the original precision.
Add strong pthread requirement to manage contexts/allocations for
converting numbers between their decNumber, string, and double formats.
Nicolas Williams