In order to avoid the cross product problem with conversion functions
(from x to y types would require x*y functions), we add flexibility
regarding the input parameters to conversion functions:

Each convertion function named after the destination type now takes all
sensible arguments by name.

int is named integer because of the reserved symbol name "int" in C.

All functions should be fully backwards compatible (existing vtcs
continue to work), but compile time checks are now effectively removed.
The conversion functions now trigger vcl errors if used incorrectly or
if conversion errors occur and no failback is provided. However, if a
failback is provided, vcl errors are only raised for usage errors.

For consistency, the conversion functions now only ever truncate if
necessary. std.round() is added for explicit rounding where required.

Existing functions which are now obsolete are marked deprecated.

Ref #2899
Ref https://github.com/varnishcache/varnish-cache/wiki/VIP12:-vmod-polymorphism-(for-type-conversions)