Improve readChar

[privat]

readChar has various issues:

• it consumes a whole line of text, truncate it at the first character and lose what remain
• is not able to read newline character (it just aborts)
• also aborts on EOF

This PR proposes that:

• readChar reads the next byte in the input (run panel or stdin) and use it as the result
• \n is not special and just return 10
• return -1 (signed 32/64 bit) on EOF

The PR also add tests for readChar, readString and read