CH04::headings, make various changes to improve logical hierarchy

ch04.asciidoc

@@ -26,7 +26,7 @@ We'll examine public keys, private keys, signatures, and hash functions
 in the following sections, and then use all of them together to describe
 the addresses used by modern Bitcoin software.
 
-==== Public Key Cryptography and Cryptocurrency
+=== Public Key Cryptography
 
 ((("keys and addresses", "overview of", "public key
 cryptography")))((("digital currencies", "cryptocurrency")))Public key
@@ -61,9 +61,6 @@ _key pair_ for convenience. However, the public key can be calculated
 from the private key, so storing only the private key is also possible.
 ====
 
-[[private_public_keys]]
-==== Private and Public Keys
-
 ((("keys and addresses", "overview of", "private and public key
 pairs")))((("elliptic curve cryptography")))((("cryptography", "elliptic
 curve cryptography")))A bitcoin wallet contains a collection of key
@@ -117,8 +114,6 @@ random can significantly reduce the security of your private key and the
 bitcoins it controls.
 ====
 
-===== Generating a private key from a random number
-
 The first and most important step in generating keys is to find a secure
 source of entropy, or randomness. Creating a bitcoin key is essentially
 the same as "Pick a number between 1 and 2^256^." The exact method you
@@ -1411,16 +1406,8 @@ versions that haven't been defined yet.  This will help make your
 software usable for many years to come even if you aren't able to add
 support for new Bitcoin features as soon as they become available.
 
-==== Key Formats
-
-((("keys and addresses", "Bitcoin addresses", "key formats")))Both
-private and public keys can be represented in a number of different
-formats. These representations all encode the same number, even though
-they look different. These formats are primarily used to make it easy
-for people to read and transcribe keys without introducing errors.
-
 [[priv_formats]]
-===== Private key formats
+==== Private key formats
 
 ((("public and private keys", "private key formats")))The private key
 can be represented in a number of different formats, all of which