feat(padic): add p-adic numbers - #845

This PR implements p-adic numbers in Form, using Flint as the backend. The structure follows the floating point system quite closely. Example program:

 * #StartPadic <prime>,<precision>
 #StartPadic 7,10
 Local F = 101/13;
 ToPadic;
 Print;
 .end

Which prints

 F =
 (4 + 6*7^2 + 4*7^3 + 2*7^4 + 5*7^5 + 3*7^6 + 1*7^8 + 2*7^9);

This is the 7-adic series representation of 101/13 up to O(7^10).

There are two other statements

1. PadicToRat; which converts p-adic numbers back to rationals. It may be worth discussing whether the existing ToRat statement from the floating-point system can be reused here, since floating-point and p-adic modes cannot be active at the same time.
2. FromPadic f; which replaces the internal (protected) padic_ function by f, exposing the underlying data:

 #$prime = 7;
 #StartPadic `$prime',10
 CFunction f;
 Symbol u,v,N;
 Local F = 101/13;
 ToPadic;
 FromPadic f;
 Print;
 .sort
 F =
 f(0,10,86915469);

The (Flint) data of the padic number can be meaningfully processed further. For example

 
 Transform f decode(12,3):base=`$prime';
 Print;
 .end
 F =
 f(0,10,4,0,6,4,2,5,3,0,1,2);

This computes the coefficients shown in the first example.

Remaining items on the to-do list:

• Update the manual.
• Add regression/coverage tests.
• Transform decode currently doesn't allow for large bases/primes. This may need to be fixed, or alternatively the FromPadic statement could be extended to support this directly, for example FromPadic f, series;.
• Should the prime number be stored in the internal padic_ function as well?

This can be discussed further at the Form workshop.

This is a squashed commit of https://github.com/cbmarini/form/tree/padic