[ABNF] Add (first draft of) input grammar.

This is the (sub)grammar for input files. It is an initial draft, written based
on the Notion page 'Leo Input File Doc/Spec'. This should be compared with the
currently implemented parser of input (i.e. .in) files.

As the Leo Reference will describe (that part has not been written yet), the
input grammar is based on the lexical grammar, i.e. an input file consists of
tokens, comments, and whitespace. However, only some tokens (compared to the
syntactic grammar for Leo code files) are used, namely the ones reachable from
the `input-file` nonterminal.

Currently (i.e. im this initial version of Leo) `input-type` is (any) `type` and
`input-expression` is just a `literal`, but these may evolve as we extend the
language (e.g. we'll probably disallow circuit types and allow tuple and array
constructions). The intent is that `input-type` will be a subset of `type` and
that `input-expression` will be a subset of `expression`.

docs/grammar/README.md

@@ -148,6 +148,7 @@ keyword = %s"address"
         / %s"char"
         / %s"console"
         / %s"const"
+        / %s"constant"
         / %s"else"
         / %s"field"
         / %s"for"
@@ -399,6 +400,7 @@ symbol = "!" / "&&" / "||"
        / "+" / "-" / "*" / "/" / "**"
        / "="
        / "(" / ")"
+       / "[" / "]"
        / "{" / "}"
        / "," / "." / ".." / ";" / ":" / "?"
        / "->" / "_"
@@ -805,7 +807,7 @@ Go to: _[function-parameter](#user-content-function-parameter)_;
 
 <a name="function-parameter"></a>
 ```abnf
-function-parameter = [ %s"public" / %s"const" ] identifier ":" type
+function-parameter = [ %s"public" / %s"constant" / %s"const" ] identifier ":" type
 ```
 
 Go to: _[identifier](#user-content-identifier), [type](#user-content-type)_;
@@ -866,4 +868,56 @@ Go to: _[format-string-close-brace](#user-content-format-string-close-brace), [f
 <a name="format-string"></a>
 ```abnf
 format-string = *format-string-element
+```
+
+
+--------
+
+
+Input Grammar
+-------------
+
+<a name="input-type"></a>
+```abnf
+input-type = type
+```
+
+Go to: _[type](#user-content-type)_;
+
+
+<a name="input-expression"></a>
+```abnf
+input-expression = literal
+```
+
+Go to: _[literal](#user-content-literal)_;
+
+
+<a name="input-item"></a>
+```abnf
+input-item = identifier ":" input-type "=" input-expression ";"
+```
+
+Go to: _[identifier](#user-content-identifier), [input-expression](#user-content-input-expression), [input-type](#user-content-input-type)_;
+
+
+<a name="input-title"></a>
+```abnf
+input-title = "[" identifier "]"
+```
+
+Go to: _[identifier](#user-content-identifier)_;
+
+
+<a name="input-section"></a>
+```abnf
+input-section = input-title *input-item
+```
+
+Go to: _[input-title](#user-content-input-title)_;
+
+
+<a name="input-file"></a>
+```abnf
+input-file = *input-section
 

docs/grammar/abnf-grammar.txt

@@ -177,6 +177,7 @@ symbol = "!" / "&&" / "||"
        / "+" / "-" / "*" / "/" / "**"
        / "="
        / "(" / ")"
+       / "[" / "]"
        / "{" / "}"
        / "," / "." / ".." / ";" / ":" / "?"
        / "->" / "_"
@@ -346,3 +347,20 @@ format-string-element = not-brace
                       / format-string-close-brace
 
 format-string = *format-string-element
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+; Input Grammar
+; -------------
+
+input-type = type
+
+input-expression = literal
+
+input-item = identifier ":" input-type "=" input-expression ";"
+
+input-title = "[" identifier "]"
+
+input-section = input-title *input-item
+
+input-file = *input-section