rework precedence in expression operators

[mccanne]

This commit reworks the precedence rules in the PEG grammar to follow the SQL rules.

[philrz]

Verified in super commit a985431.

With input data.csv:

CustomerID,Country
1,Germany
8,Spain

Before:

$ super -version
Version: v1.18.0-106-g7b6b9550

$ super -c "SELECT * FROM 'data.csv' WHERE NOT Country = 'Spain';"
[no output]

Now:

$ super -version
Version: v1.18.0-107-ga9854314

$ super -c "SELECT * FROM 'data.csv' WHERE NOT Country = 'Spain';"
{CustomerID:1.,Country:"Germany"}

Thanks @mccanne!