confusing typedefs in jsonfuncs.c

The new jsonfuncs.c has some confusing typedef scheme.  For example, it
has a bunch of definitions like this:

typedef struct getState
{
    ...
} getState, *GetState;

So GetState is a pointer to getState.  I have never seen that kind of
convention before.

This then leads to code like

GetState    state;

state = palloc0(sizeof(getState));

which has useless mental overhead.

But the fact that GetState is really a pointer isn't hidden at all,
because state is then derefenced with -> or cast from or to void*.  So
whatever abstraction might have been intended isn't there at all.  And
all of this is an intra-file interface anyway.

And to make this even more confusing, other types such as ColumnIOData
and JsonLexContext are not pointers but structs directly.

I think a more typical PostgreSQL code convention is to use capitalized
camelcase for structs, and use explicit pointers for pointers.  I have
attached a patch that cleans this up, in my opinion.