[Ur] Bug with oversized int literals and hexadecimal ones bigger than 32 bits

[Gabriel Riba]

Affected: urweb.lex

1. Oversized int literals throw an unhandled Overflow exception in SML 
Basis Int64.fromString, making the lexer fail.

Documentation: http://sml-family.org/Basis/integer.html#SIG:INTEGER.scan:VAL

Adding

handle Overflow => NONE

solves it.

2. Hexadecimal literals are defined too shortly, and refuse values 
bigger then 32 bits.

hexconst = 0x[0-9A-F]{1,8};

The actual definition with max. repetitions specifier, causes an 
oversized literal to be split in two tokens making the error ugly.

It is better to accept the whole token and check for the Overflow when 
converted, adding the exception handler as with the previous section.

(This bug is from a proposal of mine, so It was my fault)




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