fix: Grammar

A bunch of small fixes combined in a single commit:
1) Variable idenfiers live inside a different namespace than type names,
   so we don't need to check for clashes.
2) Witness names should begin with a letter. This ensures that
   witness::NAME is valid Rust.
3) Jet names include the trailing `::`, so they cannot clash with
   function names.
4) Type aliases must not redefine a builtin type such as `bool`.

src/minimal.pest

@@ -7,12 +7,12 @@ statement         =  { assignment | expression }
 expression        =  { block_expression | single_expression }
 block_expression  =  { "{" ~ (statement ~ ";")* ~ expression? ~ "}" }
 
-identifier        = @{ !builtin_type ~ ASCII_ALPHA ~ (ASCII_ALPHANUMERIC | "_")* }
+identifier        = @{ ASCII_ALPHA ~ (ASCII_ALPHANUMERIC | "_")* }
 jet               = @{ "jet::" ~ (ASCII_ALPHANUMERIC | "_")+ }
-witness_name      = @{ (ASCII_ALPHANUMERIC | "_")+ }
+witness_name      = @{ ASCII_ALPHA ~ (ASCII_ALPHANUMERIC | "_")* }
 builtin_type      = @{ ("Either" | "Option" | "bool" | "List" | unsigned_type) ~ !ASCII_ALPHANUMERIC }
 
-builtin_function  = @{ (jet | "unwrap_left" | "unwrap_right" | "for_while" | "is_none" | "unwrap" | "assert" | "panic" | "match" | "into" | "fold" | "dbg") ~ !ASCII_ALPHANUMERIC }
+builtin_function  = @{ ("unwrap_left" | "unwrap_right" | "for_while" | "is_none" | "unwrap" | "assert" | "panic" | "match" | "into" | "fold" | "dbg") ~ !ASCII_ALPHANUMERIC }
 function_name     =  { !builtin_function ~ identifier }
 typed_identifier  =  { identifier ~ ":" ~ ty }
 function_params   =  { "(" ~ (typed_identifier ~ ("," ~ typed_identifier)*)? ~ ")" }
@@ -47,7 +47,7 @@ list_bound        = @{ ASCII_DIGIT+ }
 list_type         =  { "List<" ~ ty ~ "," ~ list_bound ~ ">" }
 ty                =  { alias_name | builtin_alias | sum_type | option_type | boolean_type | unsigned_type | tuple_type | array_type | list_type }
 builtin_alias     = @{ "Ctx8" | "Pubkey" | "Message64" | "Message" | "Signature" | "Scalar" | "Fe" | "Gej" | "Ge" | "Point" | "Height" | "Time" | "Distance" | "Duration" | "Lock" | "Outpoint" | "Confidential1" | "ExplicitAsset" | "Asset1" | "ExplicitAmount" | "Amount1" | "ExplicitNonce" | "Nonce" | "TokenAmount1" }
-alias_name        =  { !builtin_alias ~ identifier }
+alias_name        =  { !builtin_type ~ !builtin_alias ~ identifier }
 type_keyword      = @{ "type" ~ !ASCII_ALPHANUMERIC }
 type_alias        =  { type_keyword ~ alias_name ~ "=" ~ ty ~ ";" }