There is special handling for SpEL expressions involving a map
and an unquoted string literal key (e.g. mymap[key1]). SpEL does
not require key1 to be quoted. This special handling which is done
in Indexer getValueRef() was not being also done in the Indexer
generateCode() method that compiles the expression. Also fixed
a problem where the key was not being compiled in a new
sub scope. Without the new scope the key expression was failing
to reload the relevant context object when it needed it.

Issue: SPR-12045

This commit enables the modulus operator to be compiled when
it is used as part of a SpEL expression.

Issue: SPR-12041

This commit allows the SpEL compiler to cope with generic methods
being used in expressions involving numeric operands. Due to the
use of unbound type variables the methods may look like they
return Object but in fact they are returning objects of a numeric
type that are suitable for compilation. The changes here ensure
the runtime types are looked at if the discovered declared types
are not providing enough information. This impacts all the
operands involving numerics (mathematical and relational).

Issue: SPR-12040

Revision of SpelCompiler support, resolving a subpackage cycle through moving CodeFlow and CompilablePropertyAccessor to the main spel package

Also contains explicit ClassLoader management, passed through StandardBeanExpressionResolver and SpelParserConfiguration to SpelCompiler lookup.

Issue: SPR-10943

With these changes an optional compiler is added for SpEL
expressions. The compiler is off by default but can be enabled
via the SpEL parser configuration object or system property
(when SpEL is embedded and parser configuration is not possible).
Not all expressions are currently handled but the common
cases are and it is an extensible compilation framework.

Issue: SPR-10943