gc: Moved incremental gc behind ifdef LFS_GC

Incremental gc, being stateful and not gc-able (ironic), was always
going to need to be conditionally compilable.

This moves incremental gc behind the LFS_GC define, so that we can focus
on the "default" costs. This cuts lfs_t in nearly half!

  lfs_t with LFS_GC:   308
  lfs_t without LFS_C: 168 (-45.5%)

This does save less code than one might expect though. We still need
most of the internal traversal/gc logic for things like block allocation
and orphan cleanup, so most of the savings is limited to the RAM storing
the incremental state:

                          code          stack          ctx
  before:                37916           2608          768
  after with LFS_CFG:    37944 (+0.1%)   2608 (+0.0%)  768 (+0.0%)
  after without LFS_CFG: 37796 (-0.3%)   2608 (+0.0%)  620 (-19.3%)

On the flip side, this does mean most of the incremental gc
functionality is still availables in the lfsr_traversal_t APIs.

Applications with more advanced gc use-cases may actually benefit from
_not_ enabling the incremental gc APIs, and instead use the
lfsr_traversal_t APIs directly.

tests/test_attrs.toml

@@ -493,6 +493,7 @@ defines.COMPACT = [false, true]
 defines.GC_FLAGS = 'LFS_GC_COMPACT'
 defines.GC_STEPS = -1
 defines.GC_COMPACT_THRESH = 'BLOCK_SIZE/2'
+if = 'LFS_IFDEF_GC(true, !COMPACT)'
 code = '''
     lfs_t lfs;
     lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
@@ -528,9 +529,11 @@ code = '''
     }
 
     // try compacting?
+    #ifdef LFS_GC
     if (COMPACT) {
         lfsr_gc(&lfs) => 0;
     }
+    #endif
 
     for (int remount = 0; remount < 2; remount++) {
         // remount?
@@ -572,6 +575,7 @@ defines.COMPACT = [false, true]
 defines.GC_FLAGS = 'LFS_GC_COMPACT'
 defines.GC_STEPS = -1
 defines.GC_COMPACT_THRESH = 'BLOCK_SIZE/2'
+if = 'LFS_IFDEF_GC(true, !COMPACT)'
 code = '''
     lfs_t lfs;
     lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
@@ -608,9 +612,11 @@ code = '''
     }
 
     // try compacting?
+    #ifdef LFS_GC
     if (COMPACT) {
         lfsr_gc(&lfs) => 0;
     }
+    #endif
 
     for (int remount = 0; remount < 2; remount++) {
         // remount?
@@ -4074,6 +4080,7 @@ defines.COMPACT = [false, true]
 defines.GC_FLAGS = 'LFS_GC_COMPACT'
 defines.GC_STEPS = -1
 defines.GC_COMPACT_THRESH = 'BLOCK_SIZE/2'
+if = 'LFS_IFDEF_GC(true, !COMPACT)'
 code = '''
     lfs_t lfs;
     lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
@@ -4110,9 +4117,11 @@ code = '''
     }
 
     // try compacting?
+    #ifdef LFS_GC
     if (COMPACT) {
         lfsr_gc(&lfs) => 0;
     }
+    #endif
 
     for (int remount = 0; remount < 2; remount++) {
         // remount?