### Two-dimensional sequences for tabular data

Tabular data from CSV or spreadsheets is treated a sequence of
raws, each line being also a sequence. This implied yet another node
type to distinguish from one-dimensional sequences. The separator ';' is
defined to separate raws within a two-dimensional sequence.

When data comes from a .xlsx file, the excel:values function can
extract a two-dimensional sequence from a sheet specifying the desired
range.

The !! operator has been added to get raws one after one as the !
operator is to be used to get items one after one.

Headers can be associated to columns in a two-dimensional
sequence.

It is convenient to be able transpose a two-dimensional sequence
with the matrix:transpose() function. For example, a list of items, one
per line, can be loaded in CSV format to obtain a vertical sequence of
items then transpose it into a one-dimensional sequence.

Arithmetics could be added in the future.