Closes GH-5917

From an engine perspective, named parameters mainly add three
concepts:

 * The SEND_* opcodes now accept a CONST op2, which is the
   argument name. For now, it is looked up by linear scan and
   runtime cached.
 * This may leave UNDEF arguments on the stack. To avoid having
   to deal with them in other places, a CHECK_UNDEF_ARGS opcode
   is used to either replace them with defaults, or error.
 * For variadic functions, EX(extra_named_params) are collected
   and need to be freed based on ZEND_CALL_HAS_EXTRA_NAMED_PARAMS.

RFC: https://wiki.php.net/rfc/named_params

Closes GH-5357.

Related to GH-5627

RFC: https://wiki.php.net/rfc/saner-numeric-strings

This removes the -1 allow_error mode from is_numeric_string functions and replaces it by
a trailing boolean out argument to preserve BC in a couple of places.

Most of the changes can be resumed to "numeric" strings which emitted a E_NOTICE now emit
a E_WARNING and "numeric" strings which emitted a E_WARNING now throw a TypeError.

This mostly affects:
 - String offsets
 - Arithmetic operations
 - Bitwise operations

Closes GH-5762

Closes GH-5850

RFC: https://wiki.php.net/rfc/string_to_number_comparison

Closes GH-3886.