Add encoding, string conversion and compression methods to PackedByteArray

[Bromeon]

Adds the following methods:

• Compression
  • compress
  • decompress
  • decompress_dynamic
• Number encoding and decoding
  • {en,de}code_u8
  • {en,de}code_u16
  • {en,de}code_u32
  • {en,de}code_u64
  • {en,de}code_s8
  • {en,de}code_s16
  • {en,de}code_s32
  • {en,de}code_s64
  • {en,de}code_half
  • {en,de}code_float
  • {en,de}code_double
  • encode_var
  • decode_var, decode_var_allow_nil
  • decode_var_size
  • has_encoded_var
• String conversions
  • get_string_from_ascii
  • get_string_from_utf8
  • get_string_from_utf16
  • get_string_from_utf32
  • get_string_from_wchar
  • hex_encode

I had to change the API around decode_var() because the raw FFI methods are incredibly hard to use correctly. Check its doc for details.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-994