rust-v0.8.0

Changes:

• Big improvements in speed for BPE (Both training and tokenization) (#165)

Fixes:

• Do not open all files directly while training (#163)
• There was a bug in ByteLevel PreTokenizer that caused offsets to be wrong if a char got split up in multiple bytes. (cf #156)
• The LongestFirst truncation strategy had a bug (#174)