py: Rework and compress second part of bytecode prelude.

This patch compresses the second part of the bytecode prelude which
contains the source file name, function name, source-line-number mapping
and cell closure information.  This part of the prelude now begins with a
single varible length unsigned integer which encodes 2 numbers, being the
byte-size of the following 2 sections in the header: the "source info
section" and the "closure section".  After decoding this variable unsigned
integer it's possible to skip over one or both of these sections very
easily.

This scheme saves about 2 bytes for most functions compared to the original
format: one in the case that there are no closure cells, and one because
padding was eliminated.

tools/mpy-tool.py

@@ -142,16 +142,6 @@ def mp_opcode_format(bytecode, ip, count_var_uint):
         ip += extra_byte
     return f, ip - ip_start
 
-def decode_uint(bytecode, ip):
-    unum = 0
-    while True:
-        val = bytecode[ip]
-        ip += 1
-        unum = (unum << 7) | (val & 0x7f)
-        if not (val & 0x80):
-            break
-    return ip, unum
-
 def read_prelude_sig(read_byte):
     z = read_byte()
     # xSSSSEAA
@@ -175,6 +165,20 @@ def read_prelude_sig(read_byte):
     S += 1
     return S, E, F, A, K, D
 
+def read_prelude_size(read_byte):
+    I = 0
+    C = 0
+    n = 0
+    while True:
+        z = read_byte()
+        # xIIIIIIC
+        I |= ((z & 0x7e) >> 1) << (6 * n)
+        C |= (z & 1) << n
+        if not (z & 0x80):
+            break
+        n += 1
+    return I, C
+
 def extract_prelude(bytecode, ip):
     def local_read_byte():
         b = bytecode[ip_ref[0]]
@@ -182,16 +186,14 @@ def extract_prelude(bytecode, ip):
         return b
     ip_ref = [ip] # to close over ip in Python 2 and 3
     n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args = read_prelude_sig(local_read_byte)
+    n_info, n_cell = read_prelude_size(local_read_byte)
     ip = ip_ref[0]
 
-    ip2, code_info_size = decode_uint(bytecode, ip)
-    ip += code_info_size
-    while bytecode[ip] != 0xff:
-        ip += 1
-    ip += 1
+    ip2 = ip
+    ip = ip2 + n_info + n_cell
     # ip now points to first opcode
     # ip2 points to simple_name qstr
-    return ip, ip2, (n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args, code_info_size)
+    return ip, ip2, (n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args)
 
 class MPFunTable:
     pass
@@ -359,7 +361,6 @@ class RawCode(object):
             print('        .qstr_block_name = %s,' % self.simple_name.qstr_id)
             print('        .qstr_source_file = %s,' % self.source_file.qstr_id)
             print('        .line_info = fun_data_%s + %u,' % (self.escaped_name, 0)) # TODO
-            print('        .locals = fun_data_%s + %u,' % (self.escaped_name, 0)) # TODO
             print('        .opcodes = fun_data_%s + %u,' % (self.escaped_name, self.ip))
             print('    },')
             print('    .line_of_definition = %u,' % 0) # TODO
@@ -583,14 +584,11 @@ def read_obj(f):
 
 def read_prelude(f, bytecode):
     n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args = read_prelude_sig(lambda: read_byte(f, bytecode))
-    l1 = bytecode.idx
-    code_info_size = read_uint(f, bytecode)
+    n_info, n_cell = read_prelude_size(lambda: read_byte(f, bytecode))
     l2 = bytecode.idx
-    for _ in range(code_info_size - (l2 - l1)):
+    for _ in range(n_info + n_cell):
         read_byte(f, bytecode)
-    while read_byte(f, bytecode) != 255:
-        pass
-    return l2, (n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args, code_info_size)
+    return l2, (n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args)
 
 def read_qstr_and_pack(f, bytecode, qstr_win):
     qst = read_qstr(f, qstr_win)