Simpify varobj children handling for C

[Vladimir Prus]

At the moment, varobj.c has three related function:

	c_name_of_child
	c_value_of_child
	c_type_of_child

Those function have a big similarity of code structure. There's some non-optimal logic
as well, for example, to get a value of a structure member c_value_of_child first
calls c_name_of_child. That one determines the index of the requested filed in the type
and returns the name. After that, c_value_of_child looks up the field using the name --
despite the fact that c_name_of_child already found the type index.

This patch introduces new function c_describe_child that can return name, value and type
at the same time. The three function mentioned above are made a tiny wrapeprs over
c_describe_child.

In addition to removing duplicated logic, this patch has another side effect -- now 
all accesses to structure fields are done using indices, and not names. This opens the door
for supporting anonymous unions.

I have a similar patch for C++ in progress, and after that I'll make 'describe_child' the only
interface and remove name_of_child/value_of_child/type_of_child completely.

I wanted to post this patch first, because combined patch will be rather huge and 
non-reviewable.

OK?

- Volodya

	Simplify access to various properties of children
	variable objects in C.
	* varobj.c (value_struct_element_index): New function.
	(c_describe_child): New function.
	(c_name_of_child, c_value_of_child)
	(c_type_of_child): Rewrite to use c_describe_child.

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