Add `Parse*` functions for user

[apstndb]

(It was discussed in GCPUG Slack #memefish channel) Requiring memefish.Parser and memefish.Lexer initialization is not user friendly, so It is better to define top-level global functions for each *Parser methods.

Currently defined methods:

• ParseStatement()
• ParseStatements()
• ParseQuery()
• ParseExpr()
• ParseType()
• ParseDDL()
• ParseDDLs()
• ParseDML()
• ParseDMLs()

Common signature will be ParseSomething(filepath, s string) (ReturnType, error).