Concentrate predicate definitions

[uliska]

oll-core defines numerous new predicates:

$ git grep "define (.*?"
internal/module-handling.ily:#(define (oll-maintainer? obj)
internal/module-handling.ily:#(define (oll-maintainers? obj)
internal/module-handling.ily:#(define (repo-url? obj)
internal/module-handling.ily:#(define (oll-version-string? obj)
internal/module-handling.ily:#(define (oll-module-list? obj)
internal/options.ily:#(define (al-or-props? obj)
internal/options.ily:#(define (alist? obj)
internal/options.ily:#(define (oll-mand-prop? obj)
internal/options.ily:#(define (oll-mand-props? obj)
internal/options.ily:#(define (oll-accepted-prop? obj)
internal/options.ily:#(define (oll-accepted-props? obj)
internal/options.ily:#(define (prop-rule? obj)
internal/options.ily:#(define (enforcement-symbol? obj)
internal/options.ily:#(define (prop-rules? obj)
internal/options.ily:#(define (empty-parens? obj)
internal/os-path.ily:#(define (this-file-compiled?)
internal/properties.ily:#(define (property-definition? obj)
internal/properties.ily:#(define (prop-list? obj)
internal/properties.ily:#(define (configuration-filter? obj)
internal/properties.ily:#(define (symbol-list-or-boolean? obj)
internal/properties.ily:#(define (symbol-or-context-mod? obj)
load/templates.ily:#(define (variable-pair? obj)
temp-package-declaration.ily:#(define (oll-maintainer? obj)
temp-package-declaration.ily:#(define (oll-maintainers? obj)
temp-package-declaration.ily:#(define (integer-string? obj)
temp-package-declaration.ily:#(define (oll-version-string? obj)
util/grob-location.ily:#(define (ly:moment<=? mom1 mom2)
util/grob-location.ily:#(define (compound? sig)

Some of them are very specific and for local use only, such as enforcement-symbol? while others define rather generic data types such as symbol-list-or-boolean?. The latter should be composed to a common file that is included at the beginning. This will help developers preventing duplicate implementation of the same thing, help documenting predicates that might be of use for end-users too, and it might help keeping an eye on development in LilyPond itself.