py: Add MICROPY_ENABLE_COMPILER and MICROPY_PY_BUILTINS_EVAL_EXEC opts.

MICROPY_ENABLE_COMPILER can be used to enable/disable the entire compiler,
which is useful when only loading of pre-compiled bytecode is supported.
It is enabled by default.

MICROPY_PY_BUILTINS_EVAL_EXEC controls support of eval and exec builtin
functions.  By default they are only included if MICROPY_ENABLE_COMPILER
is enabled.

Disabling both options saves about 40k of code size on 32-bit x86.
diff --git a/py/lexer.c b/py/lexer.c
index 6bacb9f..89ecc38 100644
--- a/py/lexer.c
+++ b/py/lexer.c
@@ -31,6 +31,8 @@
 #include "py/lexer.h"
 #include "py/runtime.h"
 
+#if MICROPY_ENABLE_COMPILER
+
 #define TAB_SIZE (8)
 
 // TODO seems that CPython allows NULL byte in the input stream
@@ -785,3 +787,5 @@
     printf("\n");
 }
 #endif
+
+#endif // MICROPY_ENABLE_COMPILER