v2.2.0

Breaking changes:

• (XLSX) Renamed type Xlsx.sst to Xlsx.SST.t
• (XLSX) Replaced function Xlsx.parse_row with 2 faster functions: parse_row_with_sst and parse_row_without_sst
• (XML) Removed type Xml.SAX.element_open. It is now inlined within Xml.SAX.node
• (XML) Removed preserve_space from multiple Xml types. Use the new preserve_space function instead.