Name the functions shared between layers

[fingolfin]

First step towards #67. No behaviour change; every array entry stays, so
sglppow and SOTGrps keep working untouched.

Extending the library currently means filling six arrays by number, ending
with the line both extension packages contain:

SELECT_SMALL_GROUPS_FUNCS[ SOTGRPS_POS ] := SELECT_SMALL_GROUPS_FUNCS[ 11 ];

Nothing says what 11 is. A survey found the tangle is smaller than the file
layout suggests — exactly three functions are used across layers, and
everything else is within one layer:

slot	name
SELECT_SMALL_GROUPS_FUNCS[11]	SMALL_GROUPS_SELECT_GENERIC
ID_GROUP_FUNCS[8]	ID_GROUP_BY_TREE
CODE_SMALL_GROUP_FUNCS[8]	CODE_SMALL_GROUP_FROM_LIB

Only the generic selection moves file, from small3 to gap/small.gi, since
it belongs to no layer and that file is read first.

SMALL_GROUP_FUNCS also held three helpers taking arguments other than
( size, i, inforec ), which no inforec ever dispatched to — they were
parked in the dispatch table. They become SMALL_GROUP_1152_1920_FROM_CODE,
SMALL_GROUP_P6_NUMBER and SMALL_GROUP_P6_EXPONENT, freeing slots 13, 22
and 23 and ending the collision between SMALL_GROUP_FUNCS[13] and the
unrelated ID_GROUP_FUNCS[13]. Each gets a DeclareGlobalName where it is
first used rather than moving, to keep the diff reviewable.

tst/small.tst asserts which slots are occupied, so its expectation changes
accordingly.

Verified

• smallgrp test suite, 0 failures in 11 files
• tst/verify-indices.g, 0 failures
• manual builds without warnings
• sglppow, 0 failures in 3 files, unmodified
• SOTGrps, 0 failures in 5 files, unmodified

The last two are the point: both reference SELECT_SMALL_GROUPS_FUNCS[11] by
number, and exercise the compatibility aliases against released code.

[codecov]

Codecov Report

❌ Patch coverage is 91.86047% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.88%. Comparing base (1dd4724) to head (47fb0b1).

Files with missing lines	Patch %	Lines
gap/small.gi	93.75%	3 Missing ⚠️
id3/idgrp3.g	66.66%	2 Missing ⚠️
id4/idgrp4.g	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   99.87%   99.88%   +0.01%     
==========================================
  Files         502      507       +5     
  Lines      338190   370982   +32792     
==========================================
+ Hits       337758   370557   +32799     
+ Misses        432      425       -7     

Files with missing lines	Coverage Δ
id10/idgrp10.g	98.87% <100.00%> (ø)
id2/idgrp2.g	71.55% <100.00%> (+0.12%)	⬆️
id6/idgrp6.g	96.15% <100.00%> (ø)
small10/smlgp10.g	98.29% <100.00%> (ø)
small11/smlgp11.g	97.96% <100.00%> (+2.03%)	⬆️
small2/smlgp2.g	85.91% <100.00%> (+0.06%)	⬆️
small3/smlgp3.g	94.79% <100.00%> (+0.17%)	⬆️
small4/smlgp4.g	91.10% <100.00%> (ø)
small6/smlgp6.g	99.67% <100.00%> (+<0.01%)	⬆️
small7/smlgp7.g	94.13% <100.00%> (ø)
... and 5 more

... and 5 files with indirect coverage changes

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