[expr] numeric equality operator

Uses string equality if either of the arguments cannot be interpreted as numeric, but if you actually want [string equal]ity, use the [eq] operator instead.

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% expr {  1 == 2  }
0
% expr {  1 == 1  }
1
% expr {  0x1f == 31  }
1
======

To show you should be careful comparing floats:
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% expr { 1 == 1.0000000000000001110223 }
1
======

Comparing with zero is a lot safer:
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expr { 0 eq 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001 }
0
======

I think Tcl works with +0 and -0.

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