wire: remove towire_double()

Before this patch we used to send `double`s over the wire by just
copying them. This is not portable because the internal represenation
of a `double` is implementation specific.

Instead of this, multiply any floating-point numbers that come from
the outside (e.g. JSONs) by 1 million and round them to integers when
handling them.

* Introduce a new param_millionths() that expects a floating-point
  number and returns it multipled by 1000000 as an integer.

* Replace param_double() and param_percent() with param_millionths()

* Previously the riskfactor would be allowed to be negative, which must
  have been unintentional. This patch changes that to require a
  non-negative number.

Changelog-None

tools/generate-wire.py

@@ -199,7 +199,6 @@ class Type(FieldSet):
         'secp256k1_ecdsa_signature',
         'secp256k1_ecdsa_recoverable_signature',
         'wirestring',
-        'double',
         'errcode_t',
         'bigsize',
         'varint',