edify: Some clean-ups to libedify.

- Remove dead declarations in expr.h: SetError(), GetError(),
  ClearError().
- Remove the declaration of Build() out of expr.h.
- Use std::unordered_map to implement RegisterFunction() and
  FindFunction(); kill FinishRegistration().
- Add a testcase for calling unknown functions.

Test: mmma bootable/recovery; recovery_component_test passes.
Change-Id: I9af6825ae677f92b22d716a4a5682f58522af03b

edify/README.md

@@ -0,0 +1,111 @@
+edify
+=====
+
+Update scripts (from donut onwards) are written in a new little
+scripting language ("edify") that is superficially somewhat similar to
+the old one ("amend").  This is a brief overview of the new language.
+
+- The entire script is a single expression.
+
+- All expressions are string-valued.
+
+- String literals appear in double quotes.  \n, \t, \", and \\ are
+  understood, as are hexadecimal escapes like \x4a.
+
+- String literals consisting of only letters, numbers, colons,
+  underscores, slashes, and periods don't need to be in double quotes.
+
+- The following words are reserved:
+
+       if    then    else   endif
+
+  They have special meaning when unquoted.  (In quotes, they are just
+  string literals.)
+
+- When used as a boolean, the empty string is "false" and all other
+  strings are "true".
+
+- All functions are actually macros (in the Lisp sense); the body of
+  the function can control which (if any) of the arguments are
+  evaluated.  This means that functions can act as control
+  structures.
+
+- Operators (like "&&" and "||") are just syntactic sugar for builtin
+  functions, so they can act as control structures as well.
+
+- ";" is a binary operator; evaluating it just means to first evaluate
+  the left side, then the right.  It can also appear after any
+  expression.
+
+- Comments start with "#" and run to the end of the line.
+
+
+
+Some examples:
+
+- There's no distinction between quoted and unquoted strings; the
+  quotes are only needed if you want characters like whitespace to
+  appear in the string.  The following expressions all evaluate to the
+  same string.
+
+     "a b"
+     a + " " + b
+     "a" + " " + "b"
+     "a\x20b"
+     a + "\x20b"
+     concat(a, " ", "b")
+     "concat"(a, " ", "b")
+
+  As shown in the last example, function names are just strings,
+  too.  They must be string *literals*, however.  This is not legal:
+
+     ("con" + "cat")(a, " ", b)         # syntax error!
+
+
+- The ifelse() builtin takes three arguments:  it evaluates exactly
+  one of the second and third, depending on whether the first one is
+  true.  There is also some syntactic sugar to make expressions that
+  look like if/else statements:
+
+     # these are all equivalent
+     ifelse(something(), "yes", "no")
+     if something() then yes else no endif
+     if something() then "yes" else "no" endif
+
+  The else part is optional.
+
+     if something() then "yes" endif    # if something() is false,
+                                        # evaluates to false
+
+     ifelse(condition(), "", abort())   # abort() only called if
+                                        # condition() is false
+
+  The last example is equivalent to:
+
+     assert(condition())
+
+
+- The && and || operators can be used similarly; they evaluate their
+  second argument only if it's needed to determine the truth of the
+  expression.  Their value is the value of the last-evaluated
+  argument:
+
+     file_exists("/data/system/bad") && delete("/data/system/bad")
+
+     file_exists("/data/system/missing") || create("/data/system/missing")
+
+     get_it() || "xxx"     # returns value of get_it() if that value is
+                           # true, otherwise returns "xxx"
+
+
+- The purpose of ";" is to simulate imperative statements, of course,
+  but the operator can be used anywhere.  Its value is the value of
+  its right side:
+
+     concat(a;b;c, d, e;f)     # evaluates to "cdf"
+
+  A more useful example might be something like:
+
+     ifelse(condition(),
+            (first_step(); second_step();),   # second ; is optional
+            alternative_procedure())