Improved functions for converting value to number.

Improved functions for converting value to number.

Added functions:
njs_value_to_number(), njs_value_to_integer(), njs_value_to_length()
njs_value_to_int32(), njs_value_to_uint32(), njs_value_to_uint16().

Removed functions:
njs_primitive_value_to_number(), njs_primitive_value_to_integer(),
njs_primitive_value_to_int32(), njs_primitive_value_to_uint32(),
njs_primitive_value_to_length().

Changed return type for njs_number_to_integer() function from int32_t
to int64_t.