Save literal value of the parsed number to preserve it for the output

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.

docs/content/manual/v1.6/manual.yml

@@ -292,11 +292,37 @@ sections:
           program can be a useful way of formatting JSON output from,
           say, `curl`.
 
+          An important point about the identity filter is that it 
+          guarantees to preserve the literal decimal representation 
+          of values. This is particularly important when dealing with numbers 
+          which can't be losslessly converted to an IEEE754 double precision 
+          representation.
+
+          jq doesn't truncate the literal numbers to double unless there
+          is a need to make arithmetic operations with the number.
+          Comparisions are carried out over the untruncated big decimal 
+          representation of the number.
+
+          jq will also try to maintain the original decimal precision of the provided
+          number literal. See below for examples. 
+
         examples:
           - program: '.'
             input: '"Hello, world!"'
             output: ['"Hello, world!"']
 
+          - program: '. | tojson'
+            input: '12345678909876543212345'
+            output: ['"12345678909876543212345"']
+
+          - program: 'map([., . == 1]) | tojson'
+            input: '[1, 1.000, 1.0, 100e-2]'
+            output: ['"[[1,true],[1.000,true],[1.0,true],[1.00,true]]"']
+
+          - program: '. as $big | [$big, $big + 1] | map(. > 10000000000000000000000000000000)'
+            input: '10000000000000000000000000000001'
+            output: ['[true, false]']
+
       - title: "Object Identifier-Index: `.foo`, `.foo.bar`"
         body: |
 
@@ -512,6 +538,16 @@ sections:
       expression that takes an input, ignores it, and returns 42
       instead.
 
+      Numbers in jq are internally represented by their IEEE754 double 
+      precision approximation. Any arithmetic operation with numbers, 
+      whether they are literals or results of previous filters, will 
+      produce a double precision floating point result.
+
+      However, when parsing a literal jq will store the original literal
+      string. If no mutation is applied to this value then it will make
+      to the output in its original form, even if conversion to double 
+      would result in a loss. 
+
     entries:
       - title: "Array construction: `[]`"
         body: |
@@ -630,6 +666,18 @@ sections:
       try to add a string to an object you'll get an error message and
       no result.
 
+      Please note that all numbers are converted to IEEE754 double precision 
+      floating point representation. Arithmetic and logical operators are working
+      with these converted doubles. Results of all such operations are also limited 
+      to the double precision. 
+
+      The only exception to this behaviour of number is a snapshot of original number 
+      literal. When a number which originally was provided as a literal is never
+      mutated until the end of the program then it is printed to the output in its 
+      original literal form. This also includes cases when the original literal 
+      would be truncated when converted to the IEEE754 double precision floating point
+      number.
+
     entries:
       - title: "Addition: `+`"
         body: |