Cache expression values to avoid recalculating them repeatedly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Currently, every expression in Kconfig files produces a new abstract
syntax tree (AST), even if it is identical to a previously encountered
one.

Consider the following code:

    config FOO
           bool "FOO"
           depends on (A || B) && C

    config BAR
           bool "BAR"
           depends on (A || B) && C

    config BAZ
           bool "BAZ"
           depends on A || B

The "depends on" lines are similar, but currently a separate AST is
allocated for each one.

The current data structure looks like this:

  FOO->dep ==> AND        BAR->dep ==> AND        BAZ->dep ==> OR
              /   \                   /   \                   /  \
            OR     C                OR     C                 A    B
           /  \                    /  \
          A    B                  A    B

This is redundant; FOO->dep and BAR->dep have identical ASTs but
different memory instances.

We can optimize this; FOO->dep and BAR->dep can share the same AST, and
BAZ->dep can reference its sub tree.

The optimized data structure looks like this:

  FOO->dep, BAR->dep ==> AND
                        /   \
         BAZ->dep ==> OR     C
                     /  \
                    A    B

This commit introduces a hash table to keep track of allocated
expressions. If an identical expression is found, it is reused.

This does not necessarily result in memory savings, as menu_finalize()
transforms expressions without freeing up stale ones. This will be
addressed later.

One optimization that can be easily implemented is caching the
expression's value. Once FOO's dependency, (A || B) && C, is calculated,
it can be cached, eliminating the need to recalculate it for BAR.

This commit also reverts commit e983b7b17ad1 ("kconfig/menu.c: fix
multiple references to expressions in menu_add_prop()").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

This clarifies the behavior of these functions.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

P_SYMBOL is a pseudo property that was previously used for data linking
purposes.

It is no longer used except for debug prints. Remove it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Move array_size.h, hashtable.h, list.h, list_types.h from scripts/kconfig/
to scripts/include/.

These headers will be useful for other host programs.

Remove scripts/mod/list.h.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

This flag is unneeded because a choice member can be detected by
other means.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Clarify that the given structures are not modified.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

E_LIST was preveously used to form an expression tree consisting of
choice members.

It is no longer used.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

P_CHOICE is a pseudo property used to link a choice with its members.

There is no more code relying on this, except for some debug code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

All users of this macro have been converted. Remove it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>