Separated cache_size out into rcache_size/pcache_size/fbuffer_size

A much requested feature, this allows much finer control of how RAM is
allocated for the system.

It was difficult to introduce this in previous versions of littlefs due
to how we steal caches during certain file operations, but now we don't
do that and treat the caches much more transparently.

Managing separate cache sizes does add a bit of code, but this is well
worth the potential for RAM savings due to increased flexibility:

           code          stack
  before: 33656           2632
  after:  33714 (+0.2%)   2640 (+0.3%)

Also interesting to note this reduces alignment requirements for the
rcache/pcache, since they don't need to share alignment, and completely
removes any alignment requirement from the file buffers.

tests/test_alloc.toml

@@ -256,8 +256,8 @@ code = '''
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'CACHE_SIZE/2',
-    '2*CACHE_SIZE',
+    'FBUFFER_SIZE/2',
+    '2*FBUFFER_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -422,8 +422,8 @@ code = '''
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'CACHE_SIZE/2',
-    '2*CACHE_SIZE',
+    'FBUFFER_SIZE/2',
+    '2*FBUFFER_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -658,8 +658,8 @@ code = '''
 [cases.test_alloc_nospc_files]
 defines.SIZE = [
     '0',
-    'CACHE_SIZE/2',
-    '2*CACHE_SIZE',
+    'FBUFFER_SIZE/2',
+    '2*FBUFFER_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',