Start using the terminology "author" and "reader" proposal

Background
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Figure 1 in doc/design.md refers to

	a) the party producing a signed checksum as "Signer", and
	b) the party verifying a signed checksum as "Verifier".

This is fine in isolation, but less appropriate when looking at it from a
broader Sigsum perspective.  For example, a "Signer" may also be a "Submitter".
It seems like we are mixing terminology for roles and concrete actors here.

The above is also ambiguous.  For example, logs and witnesses sign things;
witnesses and monitors verify things.

Proposal
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1) Replace "Signer" with "Author" when we are talking about a concrete party.

According to Wikipedia's definition (https://en.wikipedia.org/wiki/Author), an
'author is "the person who originated or gave existence to anything" and whose
authorship determines responsibility for what was created'.  This seems
appropriate for us.

The term "author" has been used in academic litterature before us for similar
purposes:
	* "In the setting of transparency logging [18] as depicted in Fig. 1, the
	author generates events intended for recipients that describe data
	processing by the author as it takes place"
	* Link to paper: https://link.springer.com/chapter/10.1007/978-3-319-45741-3_7

2) Replace "Verifier" with "Reader" when we are talking about a concrete party.

According to Wikipedia's definition (https://en.wikipedia.org/wiki/Reading),
"[r]eading is the process of taking in the sense or meaning of letters, symbols,
etc., especially by sight or touch".  Although the latter is not a perfect
description for us, the first part is quite close and we could argue that we are
in the "etc" category.

The main idea here is that it should feel intuitive that an author has readers.