Complete hedley refactoring

[jaromil]

Following the #999 merge, the internal C code refactoring is left half-way for the following zenroom types:

• big
• time
• float
• ecdh

The refactoring needs to add a ->ref counter to the structs and convert the _arg function to avoid an additional memcpy on the returned value.

The _free call needs to check the ->ref counter before freeing, which is set to one by _new

Also the pointers returned by _arg need to be flagged const and consequently all function signatures interacting with them adjusted to guard modification by callers, and enable modifications on dups only by milagro functions.

A final round of checks on milagro functions that do not modify o_arg values should be also made by hand.