New Rule: implicit_case_default -- basic implementation

[ronitnallagatla]

Implementation for implicit_case_default rule.

• Create a list of all LHS variables when you enter an always block
• Check whether case statement has an explicit default arm --> pass
• Else, check if variable in case statement has been previously assigned to, by looking at LHS variable list

TODO

• Need to test cases where multiple signals are driven in the case
• Make sure that only the signals under the always block are added to LHS vars list

Relevant Issues

• New Rule: Implicit case default dalance/svlint#268

[ShreChinno]

lgtm