Issue 9: Add backtick-quoted identifier support in lexer.nim
- Users can now write `key` to use reserved keywords as column/table names
Issue 10: Use \N as internal NULL marker instead of empty string
- isNull() now checks for \N instead of value.len == 0
- Empty strings can now be stored in NOT NULL columns
- NULL values properly distinguished from empty strings in storage, wire protocol, and HTTP JSON responses
dimgigov