add a basic structure for disassembler + executor

Instructions added
Branch and Exchange (BX)
Branch and Link (B)
Multiply and Accumulate (MUL, MLA)
Multiply Long and Accumulate (SMULL, SMLAL, UMULL, UMLAL)
Single data swap (SWP)
[WIP] Halfword Transfer (STRH, LDRH)

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

include/meson.build

@@ -1,6 +1,9 @@
 headers = files(
   'memory.hh',
-  'header.hh'
+  'bus.hh',
+  'header.hh',
 )
 
+subdir('cpu')
+
 install_headers(headers, subdir: meson.project_name(), preserve_path: true)