Move the function library into internal/functions

The FuncDef table, formula evaluation, the argument, range and
criteria helpers, decimal arithmetic and the JEV questions move out of
the engine. Functions read cells through a Reader over a small Book
interface the engine implements (Cell, Scan, Bounds, RangeAgg, Ask);
the engine finds functions through the table as before, and names
FuncDef, the remote types and the functions' errors through aliases.

An interface taking a callback would put every range read's callback
on the heap, so Book passes only values: ranges are read in chunks
into buffers the Reader keeps and reuses (Scan), and functions walk
them with callbacks that stay on the stack. A chunk evaluates no cell
past the first error, and lookups scan small chunks first, so no cell
is evaluated or visited that reading one at a time wouldn't have. The
engine fills a chunk from the occupancy index without a call per cell
(colFill), and keeps each sheet's reader between recalculations so the
buffers are made once.

docs/functions.md is generated from the new package with the same
categories and output.

commit

46e1e3a812c763df268fd54fe789fc06d7d5c1f3

parent

cb54b970fafa98bf188a6acebce6bce9e6f091c8

author

Dami <dami.etoile@gmail.com>