#1802 list indexing corrupts wide ints

[Don Porter]

Use of a Tcl_WideInt as a list index will
shimmer it to an "int".

On a 32-bit platform:
% set index [expr {(wide(1)<<31) - 2}]
2147483646
% lindex {1 2 3} $index
% incr index
2147483647
% incr index
-2147483648

Contrast with:
% set index [expr {(wide(1)<<31) - 2}]
2147483646
% incr index
2147483647
% incr index
2147483648

It's worse on 64-bit platforms, where truncation
is to an "int" even though Tcl_Integers are really
longs:
% set index [expr {wide(1)<<32}]
4294967296
% lindex {1 2 3} $index
1
% incr index
4294967297
% lindex {1 2 3} $index
2

So here, we don't lose range on the index values,
but the indexing function "wraps around".

Suspect trouble in TclGetIntForIndex

[Don Porter]

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More generally, any place that
Tcl_GetIntFromObj is called should be considered
suspect.

[Donal K. Fellows]

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I supoose we could make Tcl_GetIntFromObj know about
wideobjs...

[Donal K. Fellows]

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TclGetIntForIndex really ought to use 'long' and 'long*' not
'int' and 'int*'...