Missing core methods

[maximecb]

There are a few core methods that are currently missing but could improve the user-friendliness of Plush:

• String.split(sep)
• String.upper()
• String.lower()
• String.parse_float() // Try to parse the string as a decimal floating-point value
• String.slice(start_idx, end_idx) // Indices in bytes, end_idx is not included
• Array.slice(start_idx, end_idx) // returns a new sub-array, end_idx is not included
• Array.resize(new_size, fill_value)
• Int64.format_sep(sep_char)
  • Format an integer into a string with separators for thousands, millions, etc.
  • E.g. 1,000,000
  • This is useful for printing large numbers readably

Pull requests welcome. There are tests in the ./tests directory. Feel free to suggest other useful core methods that we could implement.