Update Fee field analysis to use unknown values

[S3v3ru5]

While tracking the max possible fee, fee field analysis only considers comparisons against int values.

e.g

1.)
txn Fee
int 10000
<=
assert

2.)
txn Fee
pushint 1000
>=
bnz fail

For the above examples, Fee field analysis is able to determine that Fee should be less than 10000 in 1 and 1000 in 2. But when the compared value is not an instruction that pushes an int value, the analysis does not consider that comparison.

False positive examples:

txn Fee
global MinTxnFee
<=
assert

txn Fee
global MinTxnFee
int 2
*
<=
assert

This PR represents non-int compared values using a single value x whose semantics are:

1. 0 < x < MAX_UINT64
2. x is greater than all the known possible max fees in the contract.

x represents an unknown bounded integer.

if the max possible fee of an execution path is x it only means that the Fee is checked to be less than or equal to some value and is bounded by some unknown integer.

[S3v3ru5]

Builds on #130