resolve errors and warnings when building with clang

[gabylb]

• remove -qascii (invalid for clang); for njsc and xlclang, it's already included in common.gypi in Node.js v12, v14 and v16
• include headers as implicit declarations are not allowed
• resolve warnings (sign compares, type-mismatch, unused variables, order of ctor init vs declaration)
• bump version to 3.1.0 (from 3.0.1)
• upgrade versions of chai and mocha

I confirmed there's no error or warning from ibm-clang build with Node.js v18.4.0, and njsc build with Node.js v14 and v16; and all 89 tests pass.

clang: https://www-40.ibm.com/servers/resourcelink/svc00100.nsf/pages/openXlCC++11ForZOs24AndZOs25?OpenDocument

[joransiu]

@gabylb : Not all the files seem to have the updated copyright year change.